Save compressed TIFF only accepts float values for dpi

[temp56817906847]

There is a trap in setting the resolution for compressed TIFF images.

# pillow v3.1.1

import PIL.Image
im = PIL.Image.new('CMYK', (100, 100))

dpi = 100.0  # works
dpi = 100  # fails

im.save('test.tif', format='TIFF',
        dpi=(dpi, dpi),
        compression='tiff_lzw',
        #compression='tiff_deflate',
        #compression='tiff_adobe_deflate'
        )

Integers fail silently resulting in the resolution value missing from the saved file. Floats work as expected.

[radarhere]

Just to note here, the reason that compression triggers the error is that compression causes libtiff to be used, and Pillow does not correctly convert the integer dpi when passing it to libtiff.

[radarhere]

#2288 is a PR to fix this and a larger swathe of issues. However, since that is not complete, I have created #3338 as a PR to just fix this.