Add test to check for existing methods while registering multiple plugins
Add test to check for existing methods while registering multiple plugins

This test differs from the existing one such as this one registers two plugins
at once. Existing one registers one plugin then registers second one later.

It was added to make sure that when registering multiple plugins at once,
method responsible for registering them will throw as well (when given
method would become overwritten).

smolak/stash-it