process_request_headers doesn't have access to options

[iilyak]

We would like to inject additional headers depending on options. In particular we have the following extract from here

def process_request_headers(headers, options) do
    case Keyword.get(options, :cookie) do
      nil ->
        headers
      cookie ->
        Keyword.put(headers, :Cookie, cookie)
    end
end

Unfortunately in order to achieve that we have to override process_arguments/3 function. Which is quite complex.

I've noticed that recently the #128 was merged. Which modifies the signature of process_request_headers to support passing body.
Would it be possible to use similar approach and implement

def process_request_headers(headers, _body, _options), do: process_request_headers(headers)

@@ -146,7 +148,7 @@ defmodule HTTPotion.Base do
        headers    = Application.get_env(:httpotion, :default_headers, [])
                     |> Keyword.merge(Keyword.get(options, :headers, [])
                       |> Enum.map(fn ({k, v}) -> { (if is_atom(k), do: k, else: String.to_atom(to_string(k))), to_string(v) } end))
-                     |> process_request_headers(body)
+                     |> process_request_headers(body, options)

[valpackett]

of course, would you like to send a PR?

// wow, couchdb test suite :)