bugfix: correctly extend options.schema

[NickBolles]

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
  I think? I can amend it if need be, let me know
• Tests for the changes have been added (for bug fixes / features)
  tested locally via direct node_modules change
• Docs have been added / updated (for bug fixes / features)
  not needed?

PR Type

What kind of change does this PR introduce?

[x] 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?

**covered in the issue: **
Issue Number: #316

What is the new behavior?

The schema is merged into the auto generated schema correctly.

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

[kamilmysliwiec]

Thank you!

[lock]

This thread has been automatically locked since there has not been any recent activity after it was closed. Please open a new issue for related bugs.