Add multipart support

[TheSmartnik]

Hi,
This pull request fixes long-standing issue, of not handling file upload properly #52. No extra dependencies introduced. Tested file upload on rails server, works alright.

[jnunemaker]

Neat. I really don't know much about multipart uploads, but this seems simple and contained so I'm going to merge it. @TheSmartnik are you ok with me pinging you in the future if questions or other pull requests come up about this?

[TheSmartnik]

@jnunemaker sure, I'd be super happy to help

[jirutka]

This implementation is broken – header Content-Type is not added when you pass any header into the request method.

The problem is that @raw_request.initialize_http_header(headers_hash) called few lines below (when options hash contains :headers) resets @header.

[TheSmartnik]

@jirutka oh, I see. Thanks a lot for feedback and pr

[wayne530]

May be worth emphasizing for many folks that decided to use httmultiparty when on httparty < 0.16 that using >= 0.16 + httmultiparty causes issues and multi-part form data isn't properly encoded.