Normalize a whitelisted set of options to symbols, to prevent a DOS by a...

... bad actor abusing Ruby's lack of garbage collection on symbols. Added tests to URI::New to cover cases where Addressable::URIs are initialized by option hashes both of strings AND symbols.

As initially proposed, this would resolve https://github.com/google/signet/pull/40 at Addressable's level by normalizing options sent on the whitelisted option list.

This is a more refined version of https://github.com/sporkmonger/addressable/pull/157