Check link content type against that listed in the link object

[tomhrr]

The "type" attribute of a link object is a hint as to the content type of a response got by requesting the link URL. Modify net.apnic.rdap.conformance.test.common.Link so that it accepts an optional content type in its constructor and checks that the response's content type matches. Since the context object supports alternative accept content types, Link will probably have to makes its own copy of the context for this to work correctly. A response that doesn't match the expected content type should only yield a warning.