Make MIME detection optional

- The `file_to_upload_io` method takes an extra `detect_mime_type`
  parameter wich defaults to `false`.
- The constant `QUERY_STRING_NORMALIZER` has been turned into a module
  method which takes `options` as a parameter and calls
  `file_to_upload_io` with the value of the option key
  `detect_mime_type` (which again defaults to `false`)
- Added tests for the new functionality
- Added tests to check that the old functionality still works.
- Updated tests for changed members.