Add new character to list of invalid characters.

[dianakhuang]

Tests exist here: https://github.com/edx/edx-proctoring/blob/master/edx_proctoring/backends/tests/test_software_secure.py#L246

[coveralls]

Coverage remained the same at 100.0% when pulling f9973bb on diana/new-invalid-char into 80a46ac on master.

[andy-armstrong]

@dianakhuang Does it make sense to write a unit test for this? Have you determined what the behavior is currently if an ! is found?

[dianakhuang]

In the current behavior, the ! is left in the payload. When we add it to this list, it is replaced by a _.

There already is a test (that I linked to) that iterates through the list of invalid characters and verifies that the invalid character is not present in the output. I don't know what other testing would be useful.

[andy-armstrong]

👍 thanks so much for tracking this down!