When a w:shd has some pattern with two colors, LO blends both to
render the paragraph background. We must compare that blended color
with the paragraph color on export to know if the user has edited it
or not. We were using the w:fill attribute to compare, but that was
incorrect.

Modified an existing unit test to check this behaviour. The unit test
had to be retouched because Word remove some redundant information
from the original .docx file when I saved it again with some
background changes.

Change-Id: Ia2f1ddc4afd2637e1d87b6eccd441c26853045c4