URL encoding should allow optional metadata

[davidlehn]

The URL param form should include optional-metadata:

- <url>?hl=<resource-hash>
+ <url>?hl=<resource-hash>:<optional-metadata>

This is needed so that you can encoded the content-type that is required to retrieve a representation with the proper hash.

As a general comment, the optional-metadata may need to allow any headers that are required to get the proper resources representation. content-type is just one of those.