Restore helperMissing for ambiguous statements

Fixes #318 (regression)
Partial fix for #783

lib/handlebars/compiler/javascript-compiler.js

@@ -551,6 +551,7 @@ JavaScriptCompiler.prototype = {
   // `knownHelpersOnly` flags at compile-time.
   invokeAmbiguous: function(name, helperCall) {
     this.aliases.functionType = '"function"';
+    this.aliases.helperMissing = 'helpers.helperMissing';
     this.useRegister('helper');
 
     var nonHelper = this.popStack();
@@ -561,7 +562,7 @@ JavaScriptCompiler.prototype = {
     var helperName = this.lastHelper = this.nameLookup('helpers', name, 'helper');
 
     this.push(
-      '((helper = ' + helperName + ' || ' + nonHelper
+      '((helper = (helper = ' + helperName + ' || ' + nonHelper + ') != null ? helper : helperMissing'
         + (helper.paramsInit ? '),(' + helper.paramsInit : '') + '),'
       + '(typeof helper === functionType ? helper.call(' + helper.callParams + ') : helper))');
   },

spec/helpers.js

@@ -440,6 +440,21 @@ describe('helpers', function() {
 
       shouldCompileTo(string, [context, helpers], "Hello <a>world</a>");
     });
+
+    it("if a value is not found, custom helperMissing is used", function() {
+      var string = "{{hello}} {{link_to}}";
+      var context = { hello: "Hello", world: "world" };
+
+      var helpers = {
+        helperMissing: function(options) {
+          if(options.name === "link_to") {
+            return new Handlebars.SafeString("<a>winning</a>");
+          }
+        }
+      };
+
+      shouldCompileTo(string, [context, helpers], "Hello <a>winning</a>");
+    });
   });
 
   describe("knownHelpers", function() {