1) Absolute paths without a hostname were being rewritten relative to
the current page, rather than relative to the current host.
e.g. When viewing /some/page.html, it would rewrite:
/other/page.html => proxyPrepend/some/other/page.html
Instead, it should rewrite:
/other/page.html => proxyPrepend/other/page.html
2) Trailing slashes were being stripped from all rewritten URLs.
This is because path.Join() always calls path.Clean() as well:
http://golang.org/pkg/path/#Join
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