Added test_is_valid_can_be_called_repeatedly() to the test set.
This test checks if is_valid() can be called multiple times.
Many implementations failed for this, because a list of digits
that was created at init time, got mangled by the Luhn
transformation, causing the second call to is_valid() to
work with different input data.
Merge pull request #136 from mmakaay/master

Added test_is_valid_can_be_called_repeatedly() to the test set.