Fix invalid test in to_bool

[dmarteau]

The value default_value is returned only if the tested vaiue is evaluated to False.
By default default_value is True then if the parameter is not given explicitely it is useless because we return the same value as the tested one.

If default_value si False:

• If the tested value is a string then default_value has no effect
• If the tested value is not a string and evaluated to False we return False
• If the tested value is not a string and evaluated to True we return False

=> This means that this test only check if the tested value is a string and not the value we want to test !

In the source code, default_value is explicitely set to False only when testing a string (from os.getenv) and so default_value has - hopefully - no effect.