Bugfix Request->dispatch_path when env->{PATH_INFO} is empty.

In #329 @xsawyerx pointed out an edge case in which Request->dispatch_path
was being incorrectly calculated. Convert `''` to `'/'` to cover this case.

Update the request tests to cover all combinations (allowable in the PSGI
spec) of SCRIPT_NAME, PATH_INFO and REQUEST_URI being empty and/or set.