[5.6] Allow integer values for whereYear() in SQLite

[staudenmeir]

whereYear() doesn't work with integer values in SQLite:

Post::whereYear('created_at', Carbon::now()->year)->get();          // Doesn't work.
Post::whereYear('created_at', (string) Carbon::now()->year)->get(); // Works.

This is caused by the strict comparison between the string result of strftime() and the integer value. We can solve it by casting the value to a string.

Although the documentation example uses a string, IMHO integers should also work (as they do in MySQL).

Adds SQLite integration tests for date queries.

Fixes #24058.