bug fix: make cached view function make_cache_key attr writeable

[snarfed]

fixes #97. this matches how memoize already does the same thing:

flask-caching/src/flask_caching/__init__.py

Line 838 in b526e03

cache_key = decorated_function.make_cache_key(f, *args, **kwargs)

Checklist:

• Add tests that demonstrate the correct behavior of the change. Tests should fail without the change.
• N/A Add or update relevant docs, in the docs folder and in code.
• Add an entry in CHANGES.rst summarizing the change and linking to the issue.
• N/A Add .. versionchanged:: entries in any relevant code docs.
• Run pre-commit hooks and fix any issues.
• Run pytest and tox, no tests failed.

[northernSage]

LGTM, thanks @snarfed