Using a map with multipart-params leads to non-determinant ordering

[calebsmith]

I was having an issue passing a map to multipart params in which the server would reject the request as invalid when the code was run in certain contexts, but not others. It seemed deterministic, but was puzzling because the code was identical.

I finally realized that multipart/form-data is supposed to be sent in the order the fields are presented in the document, per http://www.w3.org/TR/html4/interact/forms.html#h-17.13.4

Therefore, for reliable use of this, an ordered data structure is needed such as a vector or ordered-map