Fix bugs of CSS3 mediaqueries' test cases

A meaningless or invalid value should be treated as default.

Signed-off-by: xiaojunwu <xiaojunx.a.wu@intel.com>
> assert_equals: The text color expected "rgb(0, 255, 0)" but got "rgb(0, 0, 0)"

Got this error message on both Chrome Version 39.0.2150.5 dev-m (64-bit) and Firefox Nightly.

Please double check all the updated tests and attach unit test result summary to the commit message, thanks.