[12.x] Fix aggregate alias when using expression

[iamgergo]

Recently I discovered, when using withAggregate with a custom Expression insead of passing a column, the alias is not properly formatted.

For example:

$model->withAggregate('foo', new Expression('TIMESTAMPDIFF(SECOND, `created_at`, `updated_at`)'), 'sum')

// The generated alias:

// Before: foo_sum_t_i_m_e_s_t_a_m_p_d_i_f_f_s_e_c_o_n_d_created_at_updated_at

// After: foo_sum_timestampdiffsecond_created_at_updated_at

We could add an alias explicitly to handle this, but often it's easier to use the generated column aliases.

I targeted the master branch, because I think this could be a breaking change.