GetBoolean might be called on a decimal

[lauxjpn]

The following test (and 1 more) currently fails in Pomelo.MySql:
https://github.com/aspnet/EntityFrameworkCore/blob/817aaea6ca1a24a29e8e9f3a7b2dd3748956d942/test/EFCore.Specification.Tests/Query/GearsOfWarQueryTestBase.cs#L1503-L1519

It gets translated to:

SELECT COALESCE(`w0`.`IsAutomatic`, FALSE) FROM `Weapons` AS `w` LEFT JOIN `Weapons` AS `w0` ON `w`.`SynergyWithId` = `w0`.`Id` ORDER BY `w`.`Id`

It seems that COALESCE is returning a decimal for IsAutomatic, though the column is actually of type bit(1). The code expects a bool and calls GetBoolean, which then throws.

This fix allows the explicit conversion from decimal to bool within Row.GetBoolean().

This is not critical, so we don't need a quick release for this.

[bgrainger]

FWIW, the promotion of the BIT(n) data type to DECIMAL when using COALESCE is another reason why I would strongly recommend BOOL (aka TINYINT(1)) as the preferred MySQL data type for Boolean data, not BIT(1).

The PR looks good, I will merge it.