Fix multi-line insert return type

[KminekMatej]

• bug fix
• BC break yes and no - yes since the current algorithm returns ActiveRow instead of int. no since this change actually aligns the algorithm with the documented behaviour
• forum thread: https://forum.nette.org/cs/36954-nette-database-v3-2-9-phpdoc-selection-insert-nepokryva-dokumentovany-bulk-insert#p229075

Multi-line inserting wasnt returning numeric number of affected rows but ActiveRow instead (with first inserted record).

This fix checks if the $data inserted is actually a two-dimensional array and if so, returns number of affected rows instead of the ActiveRow.

Tests were added as well since this path was not guarded

[dg]

Thanks