Add cast to tuple in merge node

[rquevedo]

In merge node raise this exception becouse "detail" is a rowproxy: brewery.common.StreamRuntimeError: stream failed. reason: exception: TypeError: node: <brewery.nodes.record_nodes.MergeNode object at 0xa76790c> can only concatenate tuple (not "list") to tuple traceback File "/usr/local/lib/python2.7/dist-packages/brewery-0.8.0-py2.7.egg/brewery/streams.py", line 533, in run self.node.run() File "/usr/local/lib/python2.7/dist-packages/brewery-0.8.0-py2.7.egg/brewery/nodes/record_nodes.py", line 344, in run joined_row += detail