title: gdfs to graph and vise versa
body: <p>The following happens when I use python's <code>osmnx</code> as follows:</p>
  
  <pre><code>graph = osmnx.graph_from_polygon(Data.bbox)
  
  gdf_nodes, gdf_edges = osmnx.save_load.graph_to_gdfs(
          graph,
          nodes=True, edges=True,
          node_geometry=True,
          fill_edge_geometry=True)
  print(gdf_edges['u']) # prints the values nicely
  
  graph2 = osmnx.save_load.gdfs_to_graph(gdf_nodes, gdf_edges) # misses 'v' and 'u' column in gdf_nodes
  </code></pre>
  
  <p>Then I get the following error:</p>
  
  <pre><code>  File "C:\Users\jpelda\AppData\Local\Continuum\anaconda3\lib\site-packages\osmnx\save_load.py", line 513, in gdfs_to_graph
      G.add_edge(u=row['u'], v=row['v'], key=row['key'], **attrs)
  
  TypeError: add_edge() missing 2 required positional arguments: 'u_for_edge' and 'v_for_edge'
  </code></pre>