Adding the key lookup to Context.call_args_string
Adding the key lookup to Context.call_args_string

This change adds the proper key lookup to the call_args_string property,
thus allowing strings to be returned.

Change-Id: I55d68892c7bd8cb4d8ddf437906ceacd3cd39223
Closes-Bug: 1491092

stackforge/bandit,stackforge/bandit,chair6/bandit,pombredanne/bandit,pombredanne/bandit