NormalizeURL does not perform IDNA normalization

IDNA normalization is only performed in NormalizeURLString, but that function returns a string. If you need a url.URL, you must then parse the result of NormalizeURLString, which means you are parsing the URL yet again, which is wasteful.

As NormalizeURLString calls NormalizeURL, the IDNA normalization in the former should be moved to the later, resulting in the URL passed to NormalizeURL having its host field IDNA normalized.