Fix walk proxies (shellscape#54)

Author: jonathantneal

lib/container.js

@@ -61,10 +61,10 @@ class Container extends Node {
     }
 
     // allow users to pass a constructor, or node type string; eg. Word.
-    type = type.name && type.prototype ? type.name : type;
+    const isTypeCallable = typeof type === 'function';
 
     return this.walk((node, index) => {
-      if (node.type === type) {
+      if (isTypeCallable && node instanceof type || !isTypeCallable && node.type === type) {
         return callback.call(this, node, index);
       }
     });

test/api.js

@@ -71,7 +71,7 @@ describe('Parser → API', () => {
   it('should walk a type constructor', () => {
     let source = '/*1*/ 5px /* 2 */',
       ast = new Parser(source).parse(),
-      expected = ['1', '2'],
+      expected = ['1', ' 2 '],
       index = 0;
 
     expect(ast.first.walkComments).to.exist;