EXIF: Switch to #chomp, #chop was cutting off last char

Tests were failing for me due to the last character of EXIF data being cut of by `#chop`. Using `#chomp` fixes this.

This is on:

Version: ImageMagick 6.8.7-0 2013-10-01 Q16 http://www.imagemagick.org
Copyright: Copyright (C) 1999-2013 ImageMagick Studio LLC
Features: DPC
Delegates: bzlib freetype jng jp2 jpeg lcms ltdl png png tiff xml zlib

and

ruby 2.0.0p247 (2013-06-27 revision 41674) [x86_64-darwin12.4.0]