[5.5] Only add value to binding if it isn't an expression

[mfn]

Fixes #22213

Before (wrong SQL being generated):

>>> DB::table('test')->whereDate('col_date', DB::Raw('NOW()'))->where('col_misc', 'test')->get();
Illuminate\Database\QueryException with message 'SQLSTATE[HY000] [2002] Connection refused (SQL: select * from `test` where date(`col_date`) = NOW() and `col_misc` = NOW())'

After (correct SQL):

>>> DB::table('test')->whereDate('col_date', DB::Raw('NOW()'))->where('col_misc', 'test')->get();
Illuminate\Database\QueryException with message 'SQLSTATE[HY000] [2002] Connection refused (SQL: select * from `test` where date(`col_date`) = NOW() and `col_misc` = test)'

The ->toSql() doesn't show the problem, thus the test checks the bindings.

Without the fix, the bindings would return the expression too, which it shouldn't.

[mfn]

@dattz I believe this fixes your reported issue #22213 , can you check?