Http message cleanup

Various cleanups for the HTTP message spec, as discussed on the mailing list.
UrlInterface was the previous proposal to which I referred, and I agree that defining that would be out of scope here.  (It was larger than this spec is.)  I don't think the metadocument is the right place to mention __toString.  We already say on the setter that we accept an object that implements __toString(), right in the docblock.  string|object and then a textual description that mentions __toString() seems like the most flexible approach to me at this point unless we want to merge in the entire Url spec proposal.  (I do not.)