`Connector.prototype.isRelational` returns a function

[achrinza]

Steps to reproduce

Call new Connector(/*...*/).isRelational()

Offending line:

https://github.com/loopbackio/loopback-connector/blob/e85797318c76890c387050c39e84e09cc83214a2/lib/connector.js#L36-L37

Current Behavior

It returns itself (a function), which then requires a 2nd call to get the expected boolean value.

Expected Behavior

Connector.prototype.isRelational should strictly return a boolean.

Link to reproduction sandbox

N/A

Additional information

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