Add tests to verify toDataURL images are the correct orientation for premultiplyAlpha=false

[nkronlage]

This adds a test to verify the image returned by toDataURL with a premultipliedAlpha=false canvas is the correct orientation.

This was suggested by:
https://bugs.webkit.org/show_bug.cgi?id=156129

I ran the test on Chrome/FireFox and got:

testing 16 by 16 attributes: {"premultipliedAlpha":false}
PASS should be 0,255,0,255
PASS should be 255,0,0,255
PASS should be 0,0,255,255

And got this on Safari:

testing 16 by 16 attributes: {"premultipliedAlpha":false}
FAIL should be 0,255,0,255
at (0, 0) expected: 0,255,0,255 was 255,0,0,255
FAIL should be 255,0,0,255
at (0, 8) expected: 255,0,0,255 was 0,255,0,255
PASS should be 0,0,255,255

[CLAassistant]

All committers have signed the CLA.

[kdashg]

Thanks for this addition! I'm fine to merge it without the changes, but it'd be nice to do these fixups. Let me know which you'd prefer!

[nkronlage]

Updated the PR with changes requested. Take another look and let me know if there's anything else. Thanks!

[kdashg]

Thanks!