Moved iCCP chunk before PLTE chunk when saving as PNG

[radarhere]

Fixes #2201

[homm]

Tests?

[radarhere]

Okay, I've added a test.

[wiredfool]

So with this patch are we handling the ordering correctly for all of the chunks now?

[radarhere]

For all of the chunks that are written specifically, yes, the iCCP change covers it.

However, there is writing of arbitrary chunks at https://github.com/python-pillow/Pillow/blob/master/PIL/PngImagePlugin.py#L762. So I have added another commit to this PR to restrict that process to writing allowed chunk names, in the correct order, that are not specifically written elsewhere in the file,.