Fix missing --reject argument type

[masdeseiscaracteres]

This pull request fixes a bug causing comma-separated options in the --reject option not being properly separated.

I remembered to:

• Update or add unit tests if needed
• Update or add documentation/comments if needed
• Made sure stray files or whitespace didn't get committed
• If significant changes, branch from develop and set to merge into develop
• Read the guidelines for contributing

Changes:

• Specified type for the --reject command-line argument in the same style it is done for --accept.