* Added a trailing `\r\n` to multipart/form-data payloads. See
  https://github.com/guzzle/guzzle/pull/871
* Added a `GuzzleHttp\Pool::send()` convenience method to match the docs.
* Status codes are now returned as integers. See
  https://github.com/guzzle/guzzle/issues/881
* No longer overwriting an existing `application/x-www-form-urlencoded` header
  when sending POST requests, allowing for customized headers. See
  https://github.com/guzzle/guzzle/issues/877
* Improved query string and path URL serialization.

  * No longer double percent-encoding characters in the path or query string if
    they are already encoded.
  * Now properly encoding the supplied path to a URL object, instead of only
    encoding ' ' and '?'.
  * Now allowing many more characters to be present in the query string without
    being percent encoded. See http://tools.ietf.org/html/rfc3986#appendix-A
