Fix mask parameter for ImageEdit request

[glyuck]

mask parameter should be sent as a file (instead of a string).
It can also be nil, so don't throw an assertion error in that case.

[jamesrochabrun]

hello thanks for contributing, the PR description is a bit confusing compared to all the code on the PR, what is the bug we are solving with this changes?

[glyuck]

Sorry – I accidentally added an extra commit to the PR, but it's been removed now.

• The mask parameter in the images/edits request should be sent as a file (PNG image)
• This parameter is optional, so we should update the following condition:

      if maskData == nil {
         assertionFailure("Failed to get mask data")
      }
  

  We should also handle the optional .file parameter in MultipartFormDataEntry.