net/url: better path resolution
This includes a simplified resolvePath function and tests for all normal and abnormal path resolution examples described in RFC 3986, sections 5.4.1 and 5.4.2 [1]. Some of those examples failed before (see http://play.golang.org/p/F0ApSaXniv).
Also, parsing a reference "//foo" now works as expected. It was treated as an absolute path with very weird results (see http://play.golang.org/p/089b-_xoNe).
During path resolution, all dot segments are removed as described by the RFC.
A few existing tests had to be changed because they expected the wrong output.
Fixes #4700.
Fixes #4706.
[1] http://tools.ietf.org/html/rfc3986#section-5.4.1
R=rsc, adg, bradfitz
CC=golang-dev
https://golang.org/cl/
7203059