title: Call json.Unmarshal inside UnmarshalJSON function without causing stack overflow
body: <p>I wanted to perform some additional steps for initializing a data structure inside my implementation <code>UnmarshalJSON</code>. Calling <code>json.Unmarshal(b, type)</code> inside that implementation, naturally, causes a stack overflow.</p>
  
  <p>The JSON decoder is continiously trying to look up, if there is a custom <code>UnmarshalJSON</code> implementation which then again, calls <code>json.Unmarshal</code>.</p>
  
  <p>Is there another way to do this? Just call the underlying default implementation without causing this?</p>