title: Py2Neo Merge using multiple property keys
body: <p>I have been trying to use the merge method to create a node using the Py2Neo driver but am having issues.</p>
  
  <p>I try <code>tx.merge(a,"Person",('name','age'))</code> but get the error: TypeError: tuple</p>
  
  <p>In the merge documentation: <a href="http://py2neo.org/v4/database.html#py2neo.database.Transaction.merge" rel="nofollow noreferrer">here</a> it says "Note that multiple property keys may be specified by using a tuple." Am I missing something simple? </p>