ref function can return string

[kerryChen95]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/nestjs/nest/blob/master/CONTRIBUTING.md
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Other... Please describe:

What is the current behavior?

ref can not be correctly inspected in this case:

ref: () => Cat.name,

What is the new behavior?

As mongoose raw supportment, ref function can return a string.

Does this PR introduce a breaking change?

• Yes
• No

Other information

[kamilmysliwiec]

lgtm