Bugfix by cloning arguments before each invoke

I had Mixpanel and Customer.io integrations setup.

But due to this bug Customer.io was never receiving the `email` trait, since the Mixpanel integration clobbered the original `email` trait when it converted it to `$email`.

Fix + test included.

Note that the `_invoke` function actually had the comment correct, but the code wasn't doing what it said:
> Call a `method` on all of initialized integrations, passing clones of arguments

You may also want to add a higher level functionality test (rather than just the raw mechanics of `_invoke`)