fix [#1406]: ensure optional_plugins to be array of symbols

[salzig]

updated the ohai configuration declaration to ensure that values written to ohai[:optional_plugins] are always an array of symbols.

Related Issue

fixes #1406

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Chore (non-breaking change that does not add functionality or fix an issue)

Checklist:

• I have read the CONTRIBUTING document.
• I have run the pre-merge tests locally and they pass.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.
• All commits have been signed-off for the Developer Certificate of Origin.

[salzig]

$ asdf local ruby 2.5.5
version 2.5.5 is not installed for ruby

$ asdf local ruby 2.6.3
version 2.6.3 is not installed for ruby

Tests are failing cause the necessary ruby version isn't installed

[salzig]

bump, any chance to get this merged? Would help me a lot.

[tas50]

@salzig Sorry for the delay. I added a test to ensure that string inputs are converted to symbols and then merged it.