Failing test for unescaping plus sign

This PR adds a couple of tests on escaping/unescaping urls containing plus sign, a case that was not covered. `unscape_url("%2B")` returns a space instead of a +, so the test suite fails now.
There's a [reported bug](https://github.com/vmg/houdini/issues/10) in Houdini, so once it's addressed and updated, this test should pass.