Fix default clause

[ngxson]

Description

SQLite is good for testing our app on local machine. However, after using it for 2-3 weeks, I've discovered some problems with the connector.

Related issues

• Cannot just set "default" key for a property (like described in the official documentation): https://loopback.io/doc/en/lb2/Model-definition-JSON-file.html#properties
• Cannot use 0 or false as default value (unless we use as a string, "0" or "false")

Checklist

• [x] New tests added or existing tests modified to cover all changes
• [x] Code conforms with the style guide