Fixed the position evaluation for radial gradients

top    should mean top center (not top left)
left   should mean center left (not top left)
center should mean center center (it was wrongly evaluated to "50%"
       Safari would not show the gradient at all, SVG would place it
       in center left)

This does not affect linear gradients.

lib/compass/sass_extensions/functions/gradient_support.rb

@@ -107,9 +107,11 @@ module Compass::SassExtensions::Functions::GradientSupport
       else
         case position
         when /top|bottom/
-          "left #{position}"
+          "center #{position}"
         when /left|right/
-          "#{position} top"
+          "#{position} center"
+        when /center/
+          "center center"
         else
           position
         end