use null-constructor for "knownHelper" instead of checking for true

- the performance-tests use a non-true but truthy value for "knownHelpers"
  and some other people might do that too.

lib/handlebars/compiler/compiler.js

@@ -56,23 +56,15 @@ Compiler.prototype = {
 
     // These changes will propagate to the other compiler components
     let knownHelpers = options.knownHelpers;
-    options.knownHelpers = {
-      'helperMissing': true,
-      'blockHelperMissing': true,
-      'each': true,
-      'if': true,
-      'unless': true,
-      'with': true,
-      'log': true,
-      'lookup': true
-    };
+
+    options.knownHelpers = Object.create(null);
+    ['helperMissing', 'blockHelperMissing', 'each', 'if', 'unless', 'with', 'log', 'lookup'].forEach(name => {
+      options.knownHelpers[name] = true;
+    });
     if (knownHelpers) {
-      // the next line should use "Object.keys", but the code has been like this a long time and changing it, might
-      // cause backwards-compatibility issues... It's an old library...
-      // eslint-disable-next-line guard-for-in
-      for (let name in knownHelpers) {
-          this.options.knownHelpers[name] = knownHelpers[name];
-      }
+      Object.keys(knownHelpers).forEach(name => {
+        this.options.knownHelpers[name] = knownHelpers[name];
+      });
     }
 
     return this.accept(program);
@@ -267,7 +259,7 @@ Compiler.prototype = {
         path = sexpr.path,
         name = path.parts[0];
 
-    if (this.options.knownHelpers[name] === true) {
+    if (this.options.knownHelpers[name]) {
       this.opcode('invokeKnownHelper', params.length, name);
     } else if (this.options.knownHelpersOnly) {
       throw new Exception('You specified knownHelpersOnly, but used the unknown helper ' + name, sexpr);
@@ -370,7 +362,7 @@ Compiler.prototype = {
       let name = sexpr.path.parts[0],
           options = this.options;
 
-      if (options.knownHelpers.propertyIsEnumerable(name)) {
+      if (options.knownHelpers[name]) {
         isHelper = true;
       } else if (options.knownHelpersOnly) {
         isEligible = false;