fix validation of upload-length header in patch handler

[mitjap]

I made a mistake in #297 thinking that either Upload-Length or Upload-Defer-Length headers MUST be present. This PR fixes this.

We don't need to validate Upload-Defer-Length as it is not expected to be present. Upload-Length can ONLY be present if extension is supported, and upload length is not yet known.

[Murderlon]

Test is still failing:

   1 failing

  1) PostHandler
       send()
         events
           must not fire the EVENT_UPLOAD_COMPLETE event when upload-length is defered:
     Error: done() invoked with non-Error: {"status_code":501,"body":"creation-defer-length extension is not (yet) supported.\n"}