fix(collapse): default to css

- Default to value set from CSS for height/width

Closes #6182
Fixes #6045

Author: wesleycho

src/collapse/collapse.js

@@ -18,12 +18,12 @@ angular.module('ui.bootstrap.collapse', [])
           horizontal = !!('horizontal' in attrs);
           if (horizontal) {
             css = {
-              width: 'auto'
+              width: ''
             };
             cssTo = {width: '0'};
           } else {
             css = {
-              height: 'auto'
+              height: ''
             };
             cssTo = {height: '0'};
           }
@@ -38,9 +38,9 @@ angular.module('ui.bootstrap.collapse', [])
 
         function getScrollFromElement(element) {
           if (horizontal) {
-            return {width: element.scrollWidth + 'px'};
+            return {width: ''};
           }
-          return {height: element.scrollHeight + 'px'};
+          return {height: ''};
         }
 
         function expand() {