utils: reorganize prepareValue conditional for clarity

Prefer positive tests; group tests for specific objects.

Author: benesch

lib/utils.js

@@ -52,16 +52,16 @@ var prepareValue = function(val) {
   if(val instanceof Date) {
     return dateToString(val);
   }
-  if(typeof val === 'undefined') {
-    return null;
-  }
   if(Array.isArray(val)) {
     return arrayString(val);
   }
-  if(!val || typeof val !== 'object') {
-    return val === null ? null : val.toString();
+  if(val === null || typeof val === 'undefined') {
+    return null;
+  }
+  if(typeof val === 'object') {
+    return JSON.stringify(val);
   }
-  return JSON.stringify(val);
+  return val.toString();
 };
 
 function dateToString(date) {