[bug] use dict keys to iterate

[bug] use dict keys to iterate

We ran into a RuntimeError yesterday on this line, the callback can
modify this dict and make the iteration raise an error. Using keys
method to iterate is safer.
See
http://stackoverflow.com/questions/11941817/python-runtimeerror-dictionary-changed-size-during-iteration-how-to-avoid-th/11941855#11941855