Fix parsing for single-dash names with .upToNextOption parsing

[natecook1000]

When capturing the values for an option with a single-dash with the .upToNextOption
parsing strategy, the parser was stopping its search for values when it encountered
the "unpacked" short option candidates. This change removes the single-dash option
before looking for values, which strips those short options (e.g. -h) from
consideration.

This fixes #327.

Checklist

• I've added at least one test that validates that my change is working, if appropriate
• I've followed the code style of the rest of the project
• I've read the Contribution Guidelines
• I've updated the documentation if necessary

[natecook1000]

@swift-ci Please test