Do not treat URIs as keys

[bobjflong]

fixes https://github.com/newsdev/archieml-ruby/issues/1

I got bitten by this too. I think URLs are compelling enough to treat this as a special case. I also think that the START_KEY regex is complex enough already to warrant an extra check.

Re. contributing, do you have advice on how to add specs? Should we contribute to the repo referenced in the submodule?