JSON graph fix to handle unicode dict keys

[hagberg]

This fix allows unicode for attribute keys in the JSON graph format writers.

This code demonstrates the issue and should now work correctly

#  -*- 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)