Use toHTML vs. instanceof checks for SafeString

Allows for us to play nicely in environments such as Node that could have multiple versions of the library loaded. Also allows for implementors to provide their own behavior, provided they know what they are doing.

Fixes #886

lib/handlebars/safe-string.js

@@ -3,7 +3,7 @@ function SafeString(string) {
   this.string = string;
 }
 
-SafeString.prototype.toString = function() {
+SafeString.prototype.toString = SafeString.prototype.toHTML = function() {
   return "" + this.string;
 };
 

lib/handlebars/utils.js

@@ -53,8 +53,8 @@ export var isArray = Array.isArray || function(value) {
 
 export function escapeExpression(string) {
   // don't escape SafeStrings, since they're already safe
-  if (string instanceof SafeString) {
-    return string.toString();
+  if (string && string.toHTML) {
+    return string.toHTML();
   } else if (string == null) {
     return "";
   } else if (!string) {

spec/utils.js

@@ -25,6 +25,12 @@ describe('utils', function() {
       var string = new Handlebars.SafeString('foo<&"\'>');
       equals(Handlebars.Utils.escapeExpression(string), 'foo<&"\'>');
 
+      var obj = {
+        toHTML: function() {
+          return 'foo<&"\'>';
+        }
+      };
+      equals(Handlebars.Utils.escapeExpression(obj), 'foo<&"\'>');
     });
     it('should handle falsy', function() {
       equals(Handlebars.Utils.escapeExpression(''), '');