Generalize multiple field values to use Iterable.

This change allows retrofit to transform methods like:
    @POST
    @FormUrlEncoded
    void sendArray(@Field("order") List<String> array )
into:
 order=xxx&order=xxx

It's working for PHP arrays too, so we can use:
@Field("order[]") List<String> array, to generate the expected php array in the http request: order[]=xxx&order[]=xxx

It add to @Field parameters a similar behaviour to we already have for query parameters  (https://github.com/square/retrofit/pull/380)