Add an optional syntax for better FactoryGirl support

[Caleb Hearth]

When I type fill_form(:user), that should function as fill_form(:user, attributes_for(:user)) currently does.

When I type fill_form(:user, :name, :email), that should function the same as fill_form(:user, attributes_for(:user).slice(:name, :email)) currently does.

When I type fill_form(:user, :name, password: 'password'), that should function the same as fill_form(:user, attributes_for(:user).slice(:name).merge(password: 'password')) currently does.