fix(select): fix positioning when using option groups

references #3594

Author: rschmukler

src/components/select/select.js

@@ -1018,20 +1018,23 @@ function SelectProvider($$interimElementProvider) {
         selectNode.classList.add('md-overflow');
       }
 
+      var focusedNode = centeredNode;
+      if ((focusedNode.tagName || '').toUpperCase() === 'MD-OPTGROUP') {
+        focusedNode = optionNodes[0] || contentNode.firstElementChild || contentNode;
+        centeredNode = focusedNode;
+      }
+
       // Get the selectMenuRect *after* max-width is possibly set above
       var selectMenuRect = selectNode.getBoundingClientRect();
       var centeredRect = getOffsetRect(centeredNode);
 
+
       if (centeredNode) {
         var centeredStyle = $window.getComputedStyle(centeredNode);
         centeredRect.paddingLeft = parseInt(centeredStyle.paddingLeft, 10) || 0;
         centeredRect.paddingRight = parseInt(centeredStyle.paddingRight, 10) || 0;
       }
 
-      var focusedNode = centeredNode;
-      if ((focusedNode.tagName || '').toUpperCase() === 'MD-OPTGROUP') {
-        focusedNode = optionNodes[0] || contentNode.firstElementChild || contentNode;
-      }
 
       if (isScrollable) {
         var scrollBuffer = contentNode.offsetHeight / 2;