Remove default from 'ensurable'

This is my fault. I for some reason thought about passing a custom block
to 'ensurable'. However, it overrides the default behaviour of 'ensurable'
such as using the 'create' function. This confusion all came from my
understanding of how when a resource is labled 'managed' and how
'ensurable' behaves in response to 'managed' state.

This commit restores the proper functionality of ensurable.