Fix thumbs of CMYK PDFs for IE (credit to Serhiy)

Some pdf thumbnails appeared as a red "X" in IE
 if the pdf was using the CMYK colorspace. 
This fix was authored by Serhiy, described here: 
https://groups.google.com/forum/?fromgroups=#!topic/islandora/nulnlsscWZk
Merge pull request #30 from rosiel/7.x

Fix thumbs of CMYK PDFs for IE (credit to Serhiy)