fix ParamValidator failing on document trait (#4011)

Author: AllanZhengYP

Diff for: .changes/next-release/bugfix-ParamValidator-b021ee76.json

@@ -0,0 +1,5 @@
+{
+  "type": "bugfix",
+  "category": "ParamValidator",
+  "description": "fix the issue that the ParamValidator always fails on the document trait"
+}

Diff for: lib/param_validator.js

@@ -53,6 +53,8 @@ AWS.ParamValidator = AWS.util.inherit({
   validateStructure: function validateStructure(shape, params, context) {
     this.validateType(params, context, ['object'], 'structure');
 
+    if (shape.isDocument) return true;
+
     var paramName;
     for (var i = 0; shape.required && i < shape.required.length; i++) {
       paramName = shape.required[i];