Feature flag evaluation is case sensitive, while feature flag payload match evaluation is evaluated as lower()

[scotasaurus]

In posthog/client.py, feature flag key matches are consistently evaluated with the raw/provided key value, whereas calls to evaluate payloads call str(key).lower(). This leads to inconsistencies for callers with capitalized feature flag key names--the flag will correctly evaluate as a match, but any payloads associated with the feature flag key will be returned as 'None.'

It's unclear what the desired behavior is here--either feature flags should be all lower case, or the payload evaluation with lower() should be removed.