[issue19990] Add unittests for imghdr module

[Vajrasky Kok]

Vajrasky Kok added the comment:

Beside my review, I think we need one more test that tests the invalid image file, for example file with header b'cutecat'.

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nosy: +vajrasky

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue19990>
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