Fixed failing test and cleaned up syntax

The client code that sends a string for upload, instead of reading from a file, has been updated to pass a filename to use, following the documentation for the multipart-post gem UploadIO#initialize method: https://github.com/nicksieger/multipart-post/blob/abb54501ecb43e554723c9fa2a814a19b6348bbe/lib/composite_io.rb#L65

Also cleaned up some calls in the file_upload_test.rb file where the dot-operator and method name were separated with a newline from the object having the method invoked upon it, in a few places.

Added Gemfile and Gemfile.lock in order to pin correct gem dependency versions for developer testing.