Fixed some `.gists()` functions, and wrote tests.

[manchicken]

• All three gist star-related functions suppressed errors, which has been fixed
• .gists().is_starred()
  • 204 case covered
  • 404 case covered
  • 500 case covered
  • Now handles errors properly
• .gists().star()
  • 204 case covered
  • 404 case covered
  • 500 case covered
  • Now handles errors properly
• .gists().unstar()
  • 204 case covered
  • 304 case covered
  • 404 case covered
  • 500 case covered
  • Now handles errors properly
• .gists().delete()
  • 204 case covered
  • 304 case covered
  • 404 case covered
  • 500 case covered
  • Now handles errors properly
• Removed an unused import from src/api/checks.rs

[XAMPPRocky]

Thank you for your PR!