Use URI instead of URL to parse Java WS URLs

AsyncHttpClient eventually passes the url to URI.create anyway, so this
means that NingWSRequestHolder will fail in a manner consistent with
AsyncHttpClient.

The specific use case that this has been done to address is allowing
WebSocket URLs, which URI.create is fine with, but new URL throws an
exception.
Merge pull request #3349 from jroper/ning-uri-create

Use URI instead of URL to parse Java WS URLs