fix #501

[chaseleif]

Issue was caused when using RGB mask and requesting RGBA output.
Arrays had differing dimensions (mask did not have alpha).
Duplicate a channel and append it to the contour, this allows the subsequent math operations.

Followed instructions in CONTRIBUTING.md, flake8 and pytest.
flake8 reports 2 lines in wordcloud_cli.py (that I did not touch)
pytest passes all 76 collected items

[kitsukoe]

The fix does exactly what it says it does. It fixes the problem of contours not working in RGBA mode, and doesn't look like it will break anything else.