Update css-boxshadow.json

Bug description stated that a 0-value for "spread" for some browsers doesn't work, but it's a 0-value for "blur-radius" (optional third parameter in spec) that causes the issue, "spread-radius", the optional fourth parameter, may be 0. Confirmed on emulated version of Android 2.3.
Merge pull request #819 from malcolmmurphy/patch-1

Update css-boxshadow.json