Fix config.parse_if_needed cache invalidation

[Peter Suter]

42:ticket:5525 noted:

It appears there is an unused protected attribute _cache in [9037#file11]: tags/trac-1.0.2/trac/config.py@:293-294#L264​. Could that be removed?

But perhaps some other variable was meant to be cleared, e.g. self._sections = {} or for k in self._sections: self._sections[k]._cache = {}.

It looks like this scenario would currently fail and be fixed by that cache invalidation:

# config inherits parent.ini containing [a] b=x
config.get('a', 'b') # caches 'x' in config._sections['a']._cache['b']
# change parent.ini to [a] b=y
config.parse_if_needed() # does not invalidate that cache
config.get('a', 'b') # should return 'y' but returns cached 'x'

We should add a testcase and fix the code.