Add support for content{Encoding,MediaType} to Draft 7+

[Julian]

http://json-schema.org/latest/json-schema-validation.html#rfc.section.8.3

This looks like it was made optional.

I guess it can be off-by-default like format, or, since it's at least finite (maybe?) on by default.

Will see later.

[Julian]

Only draft7 even allowed these to be treated as assertions it seems, so I think this can be WONTFIXed. If someone ever decides to create a parallel assertion vocabulary for these it can be supported externally.