UTF-8 Encoding

[romulowspp]

Hello,

This lib is seemingly encoding data as US_ASCII. This presents an issue for my project, as I need to send data other than images using the same request.

I need an option to call the setUtf8Charset() method of the underlying Java library, to send data as UTF-8.

Tested on Android.

[romulowspp]

I open a pull request. #79