Fix overshadowing of overlapped subapp prefixes (#3701).

[nikie]

What do these changes do?

Fix for issue #3701 - overshadowing of overlapped subapp prefixes.

The fix allows different subapps to have prefixes with overlapping beginning. The following didn't work properly before - the second subapp prefix was hidden by the first one as subapp prefixes matching was performed by startswith only, so the first added app won:

    app.add_subapp('/qwe/', create_subapp01())
    app.add_subapp('/qwerty/', create_subapp02())

Are there changes in behavior for the user?

Subapps with overlapping prefixes can now be used without unexpected overshadowing.

Related issue number

#3701

Notes

• I have considered storing the value of self._prefix + '/' in an attribute for optimization but was not sure to do that because it adds complexity. Please let me know if such optimization is desired - I can dig further.

• Without adding request.url.raw_path != self._prefix to the comparison the existing behavior of matching empty and root subapp routes would have changed. That didn't break any existing test though, so I have added 2 more tests to make sure this works as before - test_urldispatch.py/test_prefixed_subapp_empty_route and test_urldispatch.py/test_prefixed_subapp_root_route

Checklist

• I think the code is well written
• Unit tests for the changes exist
• Documentation reflects the changes.
  Please let me know if any documentation change is required.
• If you provide code modification, please add yourself to CONTRIBUTORS.txt
• Add a new news fragment into the CHANGES folder

[asvetlov]

Thanks!
Sorry for the long delay