Support using regexp variables to change the scheme of an URL

[aler9]

Describe the feature

Discussed in #3012
Discussed in #3283

Regexp variables can be currently used to change the host, credentials, port, path and query of an URL, but they cannot be used to change the scheme of an URL due to a preliminary validity check that is performed on the URL.

This validity check may be moved from initialization to runtime in order to support changing everything in the URL.