Add multigraph handling to json adjacency format.

Addresses #827.
Multigraph keys were not managed correctly with the json adjacency format.
Whitespace cleanup.  Copy before pop.

Make sure we copy any data before we modify it so the input data is preserved.
Fix multigraph handling with node-link format.

Addresses #827
Whitespace cleanup
Merge pull request #828 from hagberg/827-json

Multigraph handling for JSON formats. 

Fixes #827