add a test case for "Generator is already running"

this happens with or without a callback. in real life, this would
probably be a reference or syntax error that gets swallowed.
possible fix for "Generator is already running"

note: you may want to just throw any errors that occur after the first
call because i would consider that a developer error.
Merge pull request #52 from jonathanong/generator-already-running

Fix "Generator already running" error