Add placeholders to fp wrapper. [closes #1780]

lib/fp/baseConvert.js

@@ -1,5 +1,6 @@
 var mapping = require('./mapping.js'),
-    mutateMap = mapping.mutate;
+    mutateMap = mapping.mutate,
+    placeholder = {};
 
 /**
  * The base implementation of `convert` which accepts a `util` object of methods
@@ -176,7 +177,12 @@ function baseConvert(util, name, func) {
       });
       return !result;
     });
-    return result || func;
+
+    result || (result = func);
+    if (mapping.placeholder[name]) {
+      result.placeholder = placeholder;
+    }
+    return result;
   };
 
   if (!isLib) {