add special logic to suit MySQLs "sort of native boolean" thing

[Mike Bayer]

we can turn on the supports_native_boolean flag in MySQL and most things work, because it has names "BOOL", "true", "false" as aliases for tinyint, 1 and 0, plus "is" works. But of course not everything works, we get back 1s and 0s for some things so it doesnt support "native_boolean" fully. We need to provide workarounds for it.

[Mike Bayer]

this has to be there for 1.0 as it is bad enough we've flipped around on parts of this.

[Mike Bayer]

• MySQL boolean symbols "true", "false" work again. 0.9's change in 2682 disallowed the MySQL dialect from making use of the "true" and "false" symbols in the context of "IS" / "IS NOT", but MySQL supports this syntax even though it has no boolean type. MySQL remains "non native boolean", but the :func:.true and :func:.false symbols again produce the keywords "true" and "false", so that an expression like column.is_(true()) again works on MySQL. fixes #3186

→ <<cset 7b766591b05c>>