[5.7] Allow disabling sqlite foreign keys

[SjorsO]

#26298 added the ability to enable foreign key constraints for sqlite datbase connections using a config value. This PR allows you to disable foreign key constraints by setting the config value to false.

[dakira]

Hey. In what case would this be useful? Foreign key constraints are off by default. For the next Laravel release I would change this anyway because it should be on my default (as all other databases) and the Sqlite docs state this:

Note, however, that future releases of SQLite might change so that foreign key constraints enabled by default. Careful developers will not make any assumptions about whether or not foreign keys are enabled by default but will instead enable or disable them as necessary.

So I would propose a PR to master checking for false and enabling in all other cases.

[SjorsO]

The SQLite docs say that foreign key constraints can be enabled by default by passing a flag when compiling the library, so we can't assume that foreign keys are always disabled by default for all users.

It makes sense to be that if passing true enables the constraints, that passing false would disable them.

[dakira]

You're right, forget what I said. I didn't think this through. :) This should definitely be merged.

[dakira]

@SjorsO You should also expand the test that I submitted with my PR to make sure false actually disables the FKs. Not quite sure how to test this, though. Edit: I think this is actually hard to test without an actual sqlite binary that enables FKs by default.