fix DatabaseRewinder.strategy= method

When you have `DatabaseRewinder.strategy = :truncate, only: ['foos']`,
the method `DatabaseRewinder.strategy=` seems to get an array like
`[:truncate, {:only => ['foos'}]`.
fix Cleaner#strategy= method
Merge branch 'fix_strategy_assignment_method' of github.com:eitoball/database_rewinder into eitoball-fix_strategy_assignment_method

Conflicts:
	spec/database_rewinder_spec.rb