Clarify the use of $format in delta links and next links

The specification clearly forbids clients to compose $format onto a delta or next link, and allows using the Accept header.

It does not forbid servers to encode $format in a delta or next link, which would make it impossible for the client to overrule this encoded format with an Accept header.

During the TC meeting on 04/24/2014, some members suggested concerns of changing the response format when following next links. The server can prevent this format change by encoding the initial format in the next link using $format (regardless of whether the client originally used $format or not).

Format for a delta response should not depend on the original format. However, if a client explicitly used $format, they may have done so because they couldn't use the Accept header (for whatever reason), in which case they may not be able to change the Accept header when using a delta link, in which case they would expect the delta link to include $format as well.