These methods would always return false using the previously implementation (URI#getSchemeSpecificPart is documented to *never* return `null`). They also had other implementation errors. Correct these and make sure they at least handle the basic examples provided in RFC 7230. More test cases can be added as needed.

Reduce these methods to string comparisons, and add overrides that take strings directly.
Use these new methods where we had previously switched to string comparison directly.