fix(uiGridHeader): ensure that styles are rebuilt on explicit height

Refactor setting of header heights to be more DRY and ensure that rebuildStyles
is set when an explicit height is set. Do not set existing explicit heights to
 null before recalculating height.

 Fixes: angular-ui#3394, angular-ui#3382, angular-ui#3409

src/js/core/factories/Grid.js

@@ -2065,8 +2065,8 @@ angular.module('ui.grid')
         }
 
         if (container.header || container.headerCanvas) {
-          container.explicitHeaderHeight = null;
-          container.explicitHeaderCanvasHeight = null;
+          container.explicitHeaderHeight = container.explicitHeaderHeight || null;
+          container.explicitHeaderCanvasHeight = container.explicitHeaderCanvasHeight || null;
 
           containerHeadersToRecalc.push(container);
         }
@@ -2102,6 +2102,12 @@ angular.module('ui.grid')
         var maxHeaderHeight = 0;
         var maxHeaderCanvasHeight = 0;
         var i, container;
+        var getHeight = function(oldVal, newVal){
+          if ( oldVal !== newVal){
+            rebuildStyles = true;
+          }
+          return newVal;
+        };
         for (i = 0; i < containerHeadersToRecalc.length; i++) {
           container = containerHeadersToRecalc[i];
 
@@ -2111,12 +2117,7 @@ angular.module('ui.grid')
           }
 
           if (container.header) {
-            var oldHeaderHeight = container.headerHeight;
-            var headerHeight = container.headerHeight = parseInt(gridUtil.outerElementHeight(container.header), 10);
-
-            if (oldHeaderHeight !== headerHeight) {
-              rebuildStyles = true;
-            }
+            var headerHeight = container.headerHeight = getHeight(container.headerHeight, parseInt(gridUtil.outerElementHeight(container.header), 10));
 
             // Get the "inner" header height, that is the height minus the top and bottom borders, if present. We'll use it to make sure all the headers have a consistent height
             var topBorder = gridUtil.getBorderSize(container.header, 'top');
@@ -2135,12 +2136,8 @@ angular.module('ui.grid')
           }
 
           if (container.headerCanvas) {
-            var oldHeaderCanvasHeight = container.headerCanvasHeight;
-            var headerCanvasHeight = container.headerCanvasHeight = parseInt(gridUtil.outerElementHeight(container.headerCanvas), 10);
+            var headerCanvasHeight = container.headerCanvasHeight = getHeight(container.headerCanvasHeight, parseInt(gridUtil.outerElementHeight(container.headerCanvas), 10));
 
-            if (oldHeaderCanvasHeight !== headerCanvasHeight) {
-              rebuildStyles = true;
-            }
 
             // If the header doesn't have an explicit canvas height, save the largest header canvas height for use later
             //   Explicit header heights are based off of the max we are calculating here. We never want to base the max on something we're setting explicitly
@@ -2167,15 +2164,15 @@ angular.module('ui.grid')
             maxHeaderHeight > 0 && typeof(container.headerHeight) !== 'undefined' && container.headerHeight !== null &&
             (container.explicitHeaderHeight || container.headerHeight < maxHeaderHeight)
           ) {
-            container.explicitHeaderHeight = maxHeaderHeight;
+            container.explicitHeaderHeight = getHeight(container.explicitHeaderHeight, maxHeaderHeight);
           }
 
           // Do the same as above except for the header canvas
           if (
             maxHeaderCanvasHeight > 0 && typeof(container.headerCanvasHeight) !== 'undefined' && container.headerCanvasHeight !== null &&
             (container.explicitHeaderCanvasHeight || container.headerCanvasHeight < maxHeaderCanvasHeight)
           ) {
-            container.explicitHeaderCanvasHeight = maxHeaderCanvasHeight;
+            container.explicitHeaderCanvasHeight = getHeight(container.explicitHeaderCanvasHeight, maxHeaderCanvasHeight);
           }
         }