Some APIs require fields in a particular order.

Some APIs (e.g. [Gfycat](http://gfycat.com/api)'s undocumented [upload API](http://www.reddit.com/r/gfycat/comments/20xbth/any_word_on_allowing_uploading_a_gif_through_the/) ಠ_ಠ) seem to rely on the order of fields in requests.

Using the order that keys come out of the HashMap in MultipartBody is suboptimal in this case -- it can sometimes lead to HTTP 4xx errors. I've added a list to MultipartBody so that fields can be given to MultipartEntityBuilder in the same order they were received.