fix ambiguous verify default value

[akeeman]

The default value of Client's verify argument is None, which currently resolves to True when stuff is load from an rc-file, and to False otherwise. Let's be more straight forward and let it always resolve to the same thing, and let's be safe by default. The problem was intoduced by https://github.com/ecmwf/cdsapi/commit/28ac54c3bdef69fb2c80c6ab56424a7f493f91d5

[FussyDuck]

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[EddyCMWF]

Hi, I addressed this in PR #64 . The implementation is basically the same except I did it in a separate if statement for readability puposes. Thanks for you contribution! Eddy