fix(layouts): do not replace invalid attribute values

Do not modify attribute values with default/fallback values; only update the attrs hashmap with the fallback value. This allows components using matching directives to not lose the originating value. e.g.

```html
< ui-layout layout="/api/sidebar.html" />
```

will become

```html
< ui-layout layout="/api/sidebar.html" class="layout-row" />
```

src/core/services/layout/layout.js

@@ -425,7 +425,9 @@
   function buildUpdateFn(element, className, attrs) {
     return function updateAttrValue(fallback) {
       if (!needsInterpolation(fallback)) {
-        element.attr(className, fallback);
+        // Do not modify the element's attribute value; so
+        // uses '<ui-layout layout="/api/sidebar.html" />' will not
+        // be affected. Just update the attrs value.
         attrs[attrs.$normalize(className)] = fallback;
       }
     };

src/core/services/layout/layout.spec.js

@@ -20,8 +20,10 @@ describe('layout directives', function() {
 
     it('should ignore invalid values', function() {
       var element = $compile('<div layout="humpty">Layout</div>')(pageScope);
-      expect(element.hasClass("layout-row")).toBeTruthy();
+
+      expect( element.attr('layout') ).toBe('humpty');        // original attribute value unmodified
       expect(element.hasClass('layout-humpty')).toBeFalsy();
+      expect(element.hasClass("layout-row")).toBeTruthy();    // injected className based on fallback value
     });
 
     it('should support interpolated values layout-gt-sm="{{direction}}"', function() {
@@ -37,6 +39,7 @@ describe('layout directives', function() {
       expect(element.hasClass('layout-gt-sm-column')).toBeTruthy();
     });
 
+
     /**
      * For all breakpoints,
      *  - Test percentage values