Simplify helper wrapper

This improves performance slightly and gives the wrapper function a more
descriptive name to help with debugging/profiling.

lib/handlebars/runtime.js

@@ -7,7 +7,6 @@ import {
   REVISION_CHANGES
 } from './base';
 import { moveHelperToHooks } from './helpers';
-import { wrapHelper } from './internal/wrapHelper';
 import {
   createProtoAccessControl,
   resultIsAllowed
@@ -426,14 +425,20 @@ function addHelpers(mergedHelpers, helpers, container) {
   if (!helpers) return;
   Object.keys(helpers).forEach(helperName => {
     let helper = helpers[helperName];
-    mergedHelpers[helperName] = passLookupPropertyOption(helper, container);
+    mergedHelpers[helperName] = wrapHelper(helper, container);
   });
 }
 
-function passLookupPropertyOption(helper, container) {
+function wrapHelper(helper, container) {
+  if (typeof helper !== 'function') {
+    // This should not happen, but apparently it does in https://github.com/wycats/handlebars.js/issues/1639
+    // We try to make the wrapper least-invasive by not wrapping it, if the helper is not a function.
+    return helper;
+  }
   const lookupProperty = container.lookupProperty;
-  return wrapHelper(helper, options => {
+  return function invokeHelper(/* dynamic arguments */) {
+    const options = arguments[arguments.length - 1];
     options.lookupProperty = lookupProperty;
-    return options;
-  });
+    return helper.apply(this, arguments);
+  };
 }