Gateway adds Forms implying application/json even if an add-on specifies a different contentType

[benfrancis]

STR:

• Install the Virtual Things add-on
• Add a video camera Thing (or camera Thing)

Expected:

• The Thing Description just contains the Forms provided for the video property by the add-on:

        "forms": [
          {
            "op": "readproperty",
            "href": "/media/virtual-things/index.mpd",
            "contentType": "application/dash+xml"
          },
          {
            "op": "readproperty",
            "href": "/media/virtual-things/master.m3u8",
            "contentType": "application/vnd.apple.mpegurl"
          }
        ]

Actual:

• The gateway adds additional forms which imply the video (or image) property can also be read as JSON and observed using SSE, which is not true:

        "forms": [
          {
            "op": "readproperty",
            "href": "/media/virtual-things/index.mpd",
            "contentType": "application/dash+xml"
          },
          {
            "op": "readproperty",
            "href": "/media/virtual-things/master.m3u8",
            "contentType": "application/vnd.apple.mpegurl"
          },
          {
            "href": "/things/virtual-things-20/properties/video"
          },
          {
            "href": "/things/virtual-things-20/properties/video",
            "op": [
              "observeproperty",
              "unobserveproperty"
            ],
            "subprotocol": "sse"
          }
        ]

If an add-on specifies a custom Form for an InteractionAffordance with a different contentType then we should probably not add the default application/json Forms. We need to make sure this is still possible for the add-on to specify itself if JSON really is also supported.