Fixing for an edge case in Mongrel 1.2.0.pre2

I ran into a problem with rack-rewrite blowing up when the ```env['PATH_INFO']``` variable was nil and the ```env['QUERY_STRING']``` variable was present. This happens under Mongrel 1.2.0.pre2 when hitting the root URL with a query string parameter. The ```PATH_INFO``` variable is not set and ```Rack::Rewrite::Rule#build_path_from_env``` blows up when trying to append to nil.

Small fix with a test, set to an empty string if ```env['PATH_INFO']``` is nil.