perf(extend): remove use of forEach to remove calls/closures/passing arguments

[jbedard]

I found this made $parse almost 50% faster when not cached, I assume other cases will also benefit. This also removes the "bad value context for arguments value" warning in the chrome profiler.

[IgorMinar]

otherwise this looks good to me

[jbedard]

I updated the loop to use Object.keys, this required making sure obj is truthy in the if statement as well. Might be able to remove the obj !== dst now that the loop starts at i = 1, probably does no harm either way though...