title: db.Key.from_path() does not return correct key
body: <p>I am using db.Key.from_path(Model, key_name) in several different places in my code and then call either db.get() or Model.get_by_key_name(). I noticed that these latter commands were always returning 0 items even though I knew for sure I should be getting something back. Upon closer inspection, I noticed that the db.Key.from_path() command was not returning the correct key. The key returned looks very similar, especially at the beginning, but some of the characters are different and it is about 75% shorter than the key shown in the datastore viewer. Has anyone else encountered this? Thanks.</p>
  
  <p>Here is some sample code:</p>
  
  <pre><code>class Root(db.Model):
      pass
  
  class Parent(db.Model):
      pass
  
  class MyModel(db.Model):
      pass
  
  root = Root().put()
  parent = Parent(key_name=parentname,parent=root).put()
  mymodel = MyModel(key_name=mymodelname,parent=parent).put()
  
  mymodel_k = db.Key.from_path('Parent','parentname','MyModel','mymodelname')
  mymodel = db.get(mymodel_k)
  </code></pre>
  
  <p>mymodel is None</p>