title: Retrieving single property value in Gremlin query closure works, but retrieving valueMap() fails yields Exception
body: <p>Using the Titan 1.0.0 Gremlin shell I can retrieve a single property value from an edge from within a closure. But trying to access the <code>valueMap()</code> fails with an exception.</p>
  
  <p>Works:</p>
  
  <pre><code>gremlin&gt; t.E().hasLabel("TRUSTS").has('NOT_VALID_BEFORE').each( { trustEdge -&gt; t.E().has('EDGE_GROUP_ID', trustEdge.value('EDGE_GROUP_ID')).hasNot('NOT_VALID_BEFORE').each({println it.value('EDGE_ID')}) } )
  </code></pre>
  
  <p>Yields exception (only difference is in the right most closure 'it.valueMap()' vs 'it.value('..')'):</p>
  
  <pre><code>gremlin&gt; t.E().hasLabel("TRUSTS").has('NOT_VALID_BEFORE').each( { trustEdge -&gt; t.E().has('EDGE_GROUP_ID', trustEdge.value('EDGE_GROUP_ID')).hasNot('NOT_VALID_BEFORE').each( { println it.valueMap() } ) } )
  No signature of method: com.thinkaurelius.titan.graphdb.relations.StandardEdge.valueMap() is applicable for argument types: () values: []
  Possible solutions: value(java.lang.String)
  Display stack trace? [yN]
  gremlin&gt; 
  </code></pre>
  
  <p>But it is not that in general I would be unable to get to the valueMap of the edge:</p>
  
  <pre><code>gremlin&gt; t.E().hasLabel("TRUSTS").has('NOT_VALID_BEFORE').each( { trustEdge -&gt; t.E().has('EDGE_GROUP_ID', trustEdge.value('EDGE_GROUP_ID')).hasNot('NOT_VALID_BEFORE').each( { println it } ) } )
  e[215rmh-oe094-1d05-9i0][40964296-MANAGED-&gt;12312]
  gremlin&gt; t.E('215rmh-oe094-1d05-9i0').valueMap()
  ==&gt;[MANAGE_INFORM:false, NOT_VALID_AFTER:1669873006000, MANAGE_MANAGERS:false, MANAGE_AUTHENTICATION_MEANS:true, CREATED_AT:1487683094863, RELATIONSHIP_ROLE:FAMILY_DOCTOR, MANAGE_TRUST:true, UPDATED_AT:1487683094915, MANAGE_REPRESENTATION:false, EDGE_ID:122881049, VERIFIED:true, EDGE_GROUP_ID:122881049]
  </code></pre>
  
  <p>Is this a bug or am I doing something wrong here?</p>
  
  <p>A little context, just in case the query does not even do what I think it does: 
  What I think I am doing here is looking up all edges with label "TRUSTS" that have a property NOT_VALID_BEFORE. For each of those edges I look up all edges that share the same edge group ID value and check if they also have a property NOT_VALID_BEFORE, printing those to the console that do not have the property set.</p>