Add support for X-Forwarded-Proto to werkzeug.wsgi.get_current_url.

As per [my comment on issue 106](https://github.com/mitsuhiko/werkzeug/pull/106#issuecomment-2739221), this change adds another helper method - `werkzeug.wsgi.get_url_scheme` - which is modeled after `get_host` in the same module.

Added several new tests for the new helper, as well as `get_current_url` which has been modified to call it instead of relying solely on `wsgi.url_scheme`. All tests pass including the old ones.