Fix edge cases in migrator when using fill_existing_with with a boolean value

[davidepaolotua]

This covers a very edge case when performing migrations that imply adding a not null boolean column filling previous records with the false value.
In practice when performing this kind of modification

alter table_for(SomeTable) do
 add a_column : Boolean, fill_existing_with: false

the migration fails as the fill_existing_with is not considered.
The patch fixes this particular case.

Note 1:
There is currently a way to bypass this by switching the former code to:

alter table_for(SomeTable) do
 add a_column : Boolean, fill_existing_with: "false"

(add double quotes)

Note 2:
the resulting statement is now:

UPDATE users SET admin = 'false';

Is it correct? I didn't touch the logic in the BoolColumn, but I would assume that in theory it should be without single quotes. However, it seems that the same applies to FloatColumn as well...

Note 3:
Why is the default value for fill_existing_with in the add_macro nil and not :nothing?

[jwoertink]

Good catch! Thanks for the fix. This is definitely the right direction over using "false".

As for why it's defaulted to nil instead of :nothing... I'm not sure 😅 It would seem like we could just set the default to to :nothing, but I would want to dig in a bit more to double check that wouldn't mess up something else.