Don't duplicate query string arguments.

During the course of an actual request, the Request#uri method is called more
than once. If the request's path is not relative, that method destructively
updates @path, and so parameters get duplicated each time uri is called. This
change clones the path first (and adds a lame test).
Merge pull request #189 from sullman/fix-duplicate-query-string-params

Don't duplicate query string arguments.