Avoid system locales with GTFS output formatting

[tvainika]

Summary:

Fix getSecondsAsString to return '-' (U+002D ASCII minus) instead of locale specific character such as '−' (U+2212 minus sign).

Expected behavior:

The method StopTimeFieldMappingFactory.getSecondsAsString should always return ASCII minus sign as negative sign, because the output string is used GTFS output files. Therefore system locale should not affect the method behavior or return value. This software is usually running in server environments without specific locale settings, so it might not come up often.

• [X] Run the unit tests with mvn test to make sure you didn't break anything
• [ ] Format the title like "Fix # - short description of fix and changes"
• [X] Linked all relevant issues (I didn't find any)

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