fix(http-log) properly migrate headers when multiple values are provided

[tjasko]

In Kong <=2.8, the http-log plugin supports providing multiple header values in the array, and will automatically concatenate them (delimited by , ) when sent over the wire to the defined HTTP log endpoint.

Prior to these code changes, if someone has {"some-header": ["test1", "test2"]} set as the headers on their plugin config, when migrating from 2.8 to 3.0, it’d re-write the headers config to {"some-header": "test1"}. These code changes allow to retain the same behavior as prior versions.

Given the example headers config above, the headers sent by the gateway in a manual integration test are below:

Content-Length: 1556
User-Agent: lua-resty-http/0.16.1 (Lua) ngx_lua/10020
Content-Type: application/json
some-header: test1, test2

The DB migration was manually tested from 2.8 -> 3.0. Below is the state of the plugin in 2.8 (before the migration) and in 3.0 (after the migration):

// One header defined with two values.
//
// v2.8 plugin config:
{
    "method": "POST",
    "headers": {
        "some-header": [
            "test1",
            "test2"
        ]
    },
    "timeout": 10000,
    "keepalive": 60000,
    "queue_size": 1,
    "retry_count": 10,
    "content_type": "application/json",
    "flush_timeout": 2,
    "http_endpoint": "http://127.0.0.1:8081",
    "custom_fields_by_lua": null
}
// v3.0 plugin config:
{
    "method": "POST",
    "headers": {
        "some-header": "test1, test2"
    },
    "timeout": 10000,
    "keepalive": 60000,
    "queue_size": 1,
    "retry_count": 10,
    "content_type": "application/json",
    "flush_timeout": 2,
    "http_endpoint": "http://127.0.0.1:8081",
    "custom_fields_by_lua": null
}

// One header defined with one value.
//
// v2.8 plugin config:
{
    "method": "POST",
    "headers": {
        "some-header": [
            "test1"
        ]
    },
    "timeout": 10000,
    "keepalive": 60000,
    "queue_size": 1,
    "retry_count": 10,
    "content_type": "application/json",
    "flush_timeout": 2,
    "http_endpoint": "http://127.0.0.1:8081",
    "custom_fields_by_lua": null
}
// v3.0 plugin config:
{
    "method": "POST",
    "headers": {
        "some-header": "test1"
    },
    "timeout": 10000,
    "keepalive": 60000,
    "queue_size": 1,
    "retry_count": 10,
    "content_type": "application/json",
    "flush_timeout": 2,
    "http_endpoint": "http://127.0.0.1:8081",
    "custom_fields_by_lua": null
}

// One header defined with no value.
//
// v2.8 plugin config:
{
    "method": "POST",
    "headers": {
        "some-header": {}
    },
    "timeout": 10000,
    "keepalive": 60000,
    "queue_size": 1,
    "retry_count": 10,
    "content_type": "application/json",
    "flush_timeout": 2,
    "http_endpoint": "http://127.0.0.1:8081",
    "custom_fields_by_lua": null
}
// v3.0 plugin config:
{
    "method": "POST",
    "headers": null,
    "timeout": 10000,
    "keepalive": 60000,
    "queue_size": 1,
    "retry_count": 10,
    "content_type": "application/json",
    "flush_timeout": 2,
    "http_endpoint": "http://127.0.0.1:8081",
    "custom_fields_by_lua": null
}

// One header defined with one value, another with no value.
//
// v2.8 plugin config:
{
    "method": "POST",
    "headers": {
        "some-header": {},
        "another-header": [
            "test1"
        ]
    },
    "timeout": 10000,
    "keepalive": 60000,
    "queue_size": 1,
    "retry_count": 10,
    "content_type": "application/json",
    "flush_timeout": 2,
    "http_endpoint": "http://127.0.0.1:8081",
    "custom_fields_by_lua": null
}
// v3.0 plugin config:
{
    "method": "POST",
    "headers": {
        "another-header": "test1"
    },
    "timeout": 10000,
    "keepalive": 60000,
    "queue_size": 1,
    "retry_count": 10,
    "content_type": "application/json",
    "flush_timeout": 2,
    "http_endpoint": "http://127.0.0.1:8081",
    "custom_fields_by_lua": null
}

// No headers defined.
//
// v2.8 plugin config:
{
    "method": "POST",
    "headers": null,
    "timeout": 10000,
    "keepalive": 60000,
    "queue_size": 1,
    "retry_count": 10,
    "content_type": "application/json",
    "flush_timeout": 2,
    "http_endpoint": "http://127.0.0.1:8081",
    "custom_fields_by_lua": null
}
// v3.0 plugin config:
{
    "method": "POST",
    "headers": null,
    "timeout": 10000,
    "keepalive": 60000,
    "queue_size": 1,
    "retry_count": 10,
    "content_type": "application/json",
    "flush_timeout": 2,
    "http_endpoint": "http://127.0.0.1:8081",
    "custom_fields_by_lua": null
}

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