net/url: do not percent-encode valid host characters
The code in question was added as part of allowing zone identifiers
in IPv6 literals like http://[ipv6%zone]:port/foo, in golang.org/cl/2431.
The old condition makes no sense. It refers to §3.2.1, which is the wrong section
of the RFC, it excludes all the sub-delims, which §3.2.2 (the right section)
makes clear are valid, and it allows ':', which is not actually valid,
without an explanation as to why (because we keep :port in the Host field
of the URL struct).
The new condition allows all the sub-delims, as specified in RFC 3986,
plus the additional characters [ ] : seen in IP address literals and :port suffixes,
which we also keep in the Host field.
This allows mysql://a,b,c/path to continue to parse, as it did in Go 1.4 and earlier.
This CL does not break any existing tests, suggesting the over-conservative
behavior was not intended and perhaps not realized.
It is especially important not to over-escape the host field, because
Go does not unescape the host field during parsing: it rejects any
host field containing % characters.