rtpac3depay: source caps only allow "audio/ac3", not "audio/x-ac3"

The sink caps for rtpac3pay (the AC3 payloader) have both audio/ac3 and audio/x-ac3:

  SINK template: 'sink'
    Availability: Always
    Capabilities:
      audio/ac3
      audio/x-ac3

The source caps for rtpac3depay (the AC3 depayloader) only allow audio/ac3:

  SRC template: 'src'
    Availability: Always
    Capabilities:
      audio/ac3

The source caps for the depayloader (rtpac3depay) should also include audio/x-ac3. The capability for audio/x-ac3 is important because the FFMPEG AC3 plugins only accept audio/x-ac3, not audio/ac3. Also, conceptually, the depayloader should be able to reverse any payloading that the payloader does.