adds support for inserting null values with columns

[jaredanderton]

Currently, to insert a null value (from a nil/optional value), you must do an insert where you specify every column and a value for every column in the insert (so you cannot do a partial insert). This creates overhead each time a migration alters the table structure.

This change introduces support for a partial insert with optional values.

[CLAassistant]

All committers have signed the CLA.

[kilnerm]

@jaredanderton, Thanks for your submission. If we take this PR as it currently stands we are going to need to do a new major release of SwiftKuery as the update to the enum is potentially breaking. I think this highlights a big issue with the enum as a new major is not something that we really want to do so soon after SwiftKuery 3.0.

Would you be willing to refactor your PR and simply hardcode the “NULL” into the default case for packType rather than using the queryBuilder approach? We can then get the fix released and I will work on removing the enum restrictions under issue #175 ready for our next major version release.

[jaredanderton]

@kilnerm Of course. Thank you for the input. PR has been refactored.

[kilnerm]

This is a good addition. Thanks for the rework!