Fix multipart with files in binary mode and fields including non-ASCII characters

[rdimartino]

Fixes #821

irb(main):003> HTTParty.post('http://localhost:8888', body: { user: { name: "John Doe", avatar: File.open(path) } })
=> #<HTTParty::Response:0xb070 parsed_response=nil, @response=#<Net::HTTPOK 200 OK readbody=true>, @headers={"content-length" => ["0"]}>
irb(main):004> HTTParty.post('http://localhost:8888', body: { user: { name: "John Doe", avatar: File.open(path, 'rb') } })
=> #<HTTParty::Response:0xf0f0 parsed_response=nil, @response=#<Net::HTTPOK 200 OK readbody=true>, @headers={"content-length" => ["0"]}>
irb(main):005> HTTParty.post('http://localhost:8888', body: { user: { name: "Jöhñ Ďœ", avator: File.open(path) } })
=> #<HTTParty::Response:0x10ca0 parsed_response=nil, @response=#<Net::HTTPOK 200 OK readbody=true>, @headers={"content-length" => ["0"]}>
irb(main):006> HTTParty.post('http://localhost:8888', body: { user: { name: "Jöhñ Ďœ", avator: File.open(path, 'rb') } })
=> #<HTTParty::Response:0x14d20 parsed_response=nil, @response=#<Net::HTTPOK 200 OK readbody=true>, @headers={"content-length" => ["0"]}>

I think this is safe because String#force_encoding says it "does not change the underlying bytes". Also the default mode for Files in Ruby is text data which sets the default external encoding to Encoding::UTF_8.

[jnunemaker]

First, I don't know much about multi part or encoding. Haha. So I'm glad you fixed and added test.

Thanks! Confirmed that the fix works. I do think there are still holes where mismatches could happen but this fixes one that several people have reported so lets give it a whirl.

[rdimartino]

Thanks for the speedy review!