Use underscores, not spaces, in TIFF tag kwargs.

kwargs with spaces ("x resolution", "y resolution", "resolution unit"
and "date time") are deprecated.
Merge branch 'tiff-resolution' of https://github.com/anntzer/Pillow into anntzer-tiff-resolution2
Test deprecated tags
Merge pull request #1058 from hugovk/anntzer-tiff-resolution2

Tests for PR #1044: Use underscores, not spaces, in TIFF tag kwargs