Causes SQL errors when only readonly properties are specified

[mandrewpowers]

I marked a property modified as read only in my model and went to test it by executing PUT {"modified": 1} but it threw SQL errors. After enabling DEBUG on the mysql connector I realized it just removed the column set in the statement but then the query was invalid:

loopback:connector:mysql SQL: UPDATEtasksSET WHEREid=?, params: [1]

As you can see there are no columns being SET in between SET and WHERE.

[devonatdomandtom]

I've created a PR for this. Hopefully it is accepted.

[prochac]

temporary fix:
Add to model https://github.com/clarkbw/loopback-ds-timestamp-mixin too.
So at least the updateAt property is set.

edit: loopback:connector:postgresql fails too