fix spawners on bukkit

[duplexsystem]

Pull Request

Description

Changelog

Checklist

Mandatory checks

• The base branch of this PR is an unreleased version branch (that has a ver/ prefix)
  or is a branch that is intended to be merged into a version branch.
• There are no already existing PRs that provide the same changes.
• The PR is within the scope of Terra (i.e. is something a configurable terrain generator should be doing).
• Changes follow the code style for this project.
• I have read the CONTRIBUTING.md
  document in the root of the git repository.

Types of changes

• Bug Fix
• Build system
• Documentation
• New Feature
• Performance
• Refactoring
• Repository
• Revert
• Style
• Tests
• Translation

Compatibility

• Introduces a breaking change
• Introduces new functionality in a backwards compatible way.
• Introduces bug fixes

Documentation

• My change requires a change to the documentation.
• I have updated the documentation accordingly.

Testing

• I have added tests to cover my changes.
• All new and existing tests passed.

Licensing

• I am the original author of this code, and I am willing to
  release it under GPLv3.
• I am not the original author of this code, but it is in public domain or
  released under GPLv3 or a compatible license.