make test more lax so it accepts either 0px 0px or 0% 0% as valid default values for background-position
only try to normalize the units to px on IE if it's not directly set on the style attribute
Merge branch 'gonchuki-fix_getStyle_IE_units' closes #2417
Merge pull request #2556 from gonchuki/fix_background_position_test

make test more lax so it accepts either 0px 0px or 0% 0% as valid defaul...