Fixed: related x_resolution, y_resolution

Fixed: test_density(Image_Attributes_UT) in Image_attributes.rb fails #76
Fixed: test_x_resolution(Image_Attributes_UT) in Image_attributes.rb fails #81
Fixed: test_y_resolution(Image_Attributes_UT) in Image_attributes.rb fails #82

From ImageMagick 6.7.9-0:

> Initialize image->x_resolution and y_resolution to 0 in image.c (previously they were initialized to DefaultResolution, which is 72.0).

<cite>[ImageMagick: Changelog](http://www.imagemagick.org/script/changelog.php)</cite>