Fix TypeErrors in MatchMixin

[jemrobinson]

Contributor Checklist:

• I have updated the release notes at docs/source/NEWS.rst
• I have updated the automated tests.
• All tests pass on your local dev environment. See CONTRIBUTING.rst.

Summary

When MatchMixin makes calls to str.startswith, str.endswith and str.find, the argument is currently a bytes not a str. This causes a crash, which can most easily be seen when using a wildcard search filter.

decode()ing the bytes to a str fixes this issue.