title: Error when set collection dynamically
body: <p>Blow code is minor modification of :<a href="https://www.arangodb.com/tutorials/tutorial-python/" rel="nofollow">https://www.arangodb.com/tutorials/tutorial-python/</a></p>
  
  <pre><code>doc = {"_key": 'some_nice_key', "name": 'Robert', "age": '56'}
  bindVars = {"doc": doc,"collection":"test"}
  aql = "INSERT @doc INTO @collection LET newDoc = NEW RETURN newDoc"
  queryResult = db.AQLQuery(aql, bindVars = bindVars)
  </code></pre>
  
  <p>,I just made some modification:</p>
  
  <pre><code>{"doc": doc}  -&gt; {"doc": doc,"collection":"test"}
  INSERT @doc INTO users... -&gt; INSERT @doc INTO @collection...
  </code></pre>
  
  <p>it gives me error:</p>
  
  <pre><code>bind parameter 'bind parameter 'collection' has an invalid value or type near '@collection LET newDoc = NEW RET...' at position 1:18' has an invalid value or type (while parsing). Errors: {u'code': 400, u'errorNum': 1553, u'errorMessage': u"bind parameter 'bind parameter 'collection' has an invalid value or type near '@collection LET newDoc = NEW RET...' at position 1:18' has an invalid value or type (while parsing)", u'error': True}
  </code></pre>
  
  <p>Doesn't aql support set collection dynamically or what mistake have I made?</p>