Specifying the 'json' format when requesting the Metadata Document URL returns an error

Problem/Motivation

When requesting the Metadata Document URL in the json format (odata.svc/$metadata?$format=json), I receive the following error (in the json format no less):

{
    "error": {
        "message": {
            "lang": "en-US", "value": "Unsupported media type requested."
        }
    }
}

According to http://www.odata.org/media/30002/OData.html#metadatadocumentrequest, the $format variable should be respected.

Proposed resolution

All supported formats should also be supported on the Metadata Document URL.

Remaining tasks

Look into why the $format variable is not supported on the Metadata Document URL.