Sending complex UTF-8 strings provoke losing information.

For example:
Sending the field:

fields:{
'data' : JSON.stringify( { 'info' : { 'name': "Còmplex Nâmïng" } } )
}

Make some characters get lost due to the writeBytes conversion in FileUploadModule.java#102 file. This can be solved as I did in the PR #35.