Historically we have declined to try to provide real support for URLs
that contain %2F in the path, but they seem to be popping up more
often, especially in (arguably ill-considered) REST APIs that shoehorn
entire paths into individual path elements.

The obvious thing to do is to introduce a URL.RawPath field that
records the original encoding of Path and then consult it during
URL.String and URL.RequestURI. The problem with the obvious thing
is that it breaks backward compatibility: if someone parses a URL
into u, modifies u.Path, and calls u.String, they expect the result
to use the modified u.Path and not the original raw encoding.

Split the difference by treating u.RawPath as a hint: the observation
is that there are many valid encodings of u.Path. If u.RawPath is one
of them, use it. Otherwise compute the encoding of u.Path as before.

If a client does not use RawPath, the only change will be that String
selects a different valid encoding sometimes (the original passed
to Parse).

This ensures that, for example, HTTP requests use the exact
encoding passed to http.Get (or http.NewRequest, etc).

Also add new URL.EscapedPath method for access to the actual
escaped path. Clients should use EscapedPath instead of
reading RawPath directly.

All the old workarounds remain valid.

Fixes #5777.
Might help #9859.
Fixes #7356.
Fixes #8767.
Fixes #8292.
Fixes #8450.
Fixes #4860.
Fixes #10887.
Fixes #3659.
Fixes #8248.
Fixes #6658.
Reduces need for #2782.

Change-Id: I77b88f14631883a7d74b72d1cf19b0073d4f5473
Reviewed-on: https://go-review.googlesource.com/11302Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>