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 issue 4700.

Fixes issue 4706.

[1] http://tools.ietf.org/html/rfc3986#section-5.4.1

R=rsc, adg, bradfitz
CC=golang-dev
https://codereview.appspot.com/7203059

Committer: Russ Cox <rsc@golang.org>