JSON graph fix to handle unicode dict keys

This fix allows unicode for attribute keys in the JSON graph format writers.

This code demonstrates the issue and should now work correctly
```python
#  -*- coding: utf-8 -*-
import json
import networkx as nx
from networkx.readwrite import json_graph
G = nx.Graph()
q = u"qualité"
G.add_node(1, {q:q})
s = json_graph.node_link_data(G)
output = json.dumps(s, ensure_ascii=False)
data = json.loads(output)
H = json_graph.node_link_graph(data)
```