Support JSON Merge Patch semantics

[jbowes]

PATCH is commonly used to do partial updates of resources, particularly via JSON Merge Patch.

To do a PATCH like this, we need to support:

• sending a value on a field to update that field
• omitting a field entirely, to skip changing its value
• sending an explicit JSON null for a field, to unset it

Implement support for this.

If the resource to be patched looks like:

type Cat struct {
    Name     string `json:"name"`
    Nickname string `json:"nickname"`
}

Then the payload for patching should look like:

type UpdateCat struct {
    Name *string `json:"name,omitempty"`
    Nickname *string `json:"nickname,omitempty"`
}

// NullString is a magic value used for explicitly sending null in JSON Merge Patch requests
var NullString *string

// String returns a pointer to the provided string
func String(s string) *string { return &s }

UpdateCat will then need a MarshalJSON func, which is aware of NullString, and can explicitly send null when Name or Nickname == NullString.

Change the value of Name, and unset Nickname:

uc := UpdateCat{
    Name: pkg.String("Mittens"),
    Nickname: pkg.NullString,
}
// pass in to API call

Change the value of Nickname, but leave Name as is:

uc := UpdateCat{NickName: pkg.String("Little Fuzz Monster")}
// pass in to API call