Monthly heatmap improperly defaults to Active returns

[kartiksubbarao]

[copied from https://github.com/ranaroussi/quantstats/issues/278]

Due to lines like this in reports.py:
active = kwargs.get("active_returns", "False")
The active variable is incorrectly set to "False" instead of False.

This produces the wrong logic with a line like this in wrappers.py:
if active and benchmark is not None:

The fix is to change these lines in reports.py to:
active = kwargs.get("active_returns", False)