Support HTTP proxy (username, password) authentication

[half0wl]

I noticed that the Proxy-Authorization header was being inserted as a value in the Authorization: .. header when using RequestAuth.Proxy(..).

1. Using requests.RequestAuth.Proxy:

@ val reqProxy = requests.RequestAuth.Proxy("username", "password")
reqProxy: requests.RequestAuth.Proxy = Proxy("username", "password")

@ val res = requests.get("http://localhost:8881", auth=reqProxy)
requests.TimeoutException: Request to http://localhost:8881 timed out. (readTimeout: 10000, connectTimout: 10000)
...<snip, irrelevant>

2. Locally-received values using nc:

 learn-scala-hands-on ツ nc -l 8881
GET / HTTP/1.1
User-Agent: requests-scala
Accept-Encoding: gzip, deflate
Accept: */*
Authorization: Proxy-Authorization dXNlcm5hbWU6cGFzc3dvcmQ=  # <-- note this
Cache-Control: no-cache
Pragma: no-cache
Host: localhost:8881
Connection: keep-alive

Proxy-Authorization is a standalone header [0], and should not be passed in Authorization.

This PR fixes it by replacing RequestAuth.header with a RequestAuth.credentials that returns the built credential to be passed as a header value into the connection. In the Requester, it performs a match on RequestAuth.{TYPE} to determine the header key/value to set.

Unfortunately, I'm unable to get unit tests for proxy auth working - most services similar to httpbin (including it) doesn't echo the Proxy-Authorization value back, according to specs [1] [2]. We can spin up a local http server within the test environment to echo stuff back, though (it'd also reduce CI runtime if it no longer relies on external network requests). I'll leave that to you folks; for now I've commented out the test case and left some notes in there - if you switch to a locally-managed test env, uncommenting and pointing the request to the internal service should make the test pass :)

I manually tested the proxy auth portion against localhost:

(fix-proxy-auth ±) requests-scala ✖ nc -l 8881
GET / HTTP/1.1
User-Agent: requests-scala
Accept-Encoding: gzip, deflate
Accept: */*
Proxy-Authorization: Basic Zm9vOmJhcg==
Cache-Control: no-cache
Pragma: no-cache
Host: localhost:8881
Connection: keep-alive

[0] https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Proxy-Authorization [1] https://tools.ietf.org/html/rfc7235#section-4.4 [2] postmanlabs/httpbin#465

(p/s: I'm very new to Scala, would really love any and all feedback - thank you!)

[half0wl]

I've updated this to introduce a proxyAuth param instead of stuffing it into RequestAuth. I think that's a clearer implementation, but is a BC break since it removes RequestAuth.Proxy (which doesn't work with the current implementation, and it's also a security concern: dumping Proxy-Authorization .. as a value into Authorization allows the value to be captured by unintended servers).

(tl;dr this removes RequestAuth.Proxy in favor of an explicit proxyAuth parameter)

It's also worth noting that python-requests implements proxy auth through a connection URL (http[s]://[user]:[pass]@[host]:[port]) directly - it extracts the [user]:[pass] which gets b64-encoded and passed along in Proxy-Authorization: Basic <b64e>. That's a friendlier mechanism, IMO.

Tested locally:

// No proxy:
requests.get("http://httpbin.org/ip").text()
// => { "origin": "my_ip" }

// Auth required:
requests.get("http://httpbin.org/ip", proxy=("proxy.provider.com", 8011))
// => requests.RequestFailedException: Request to http://httpbin.org/ip failed with status code 407

// Incorrect auth:
requests.get("http://httpbin.org/ip", proxy=("proxy.provider.com", 8011), proxyAuth=("blah", ""))
// => requests.RequestFailedException: Request to http://httpbin.org/ip failed with status code 407

// Ok, proxied:
requests.get("http://httpbin.org/ip", proxy=("proxy.provider.com", 8011), proxyAuth=("user", "pass")).text()
// => { "origin": "proxy_ip" }

I also noticed that the JVM is removing the Proxy-Authorization header for HTTPS requests, unless jdk.http.auth.tunneling.disabledSchemes is set to "". I should probably flag that behaviour in the readme.

@lihaoyi let me know if you'd like credentials to a proxy service to reproduce/test this further

edit: fixes #79