Fix: Escape date/time separators

[zornwal]

Escape date and time separators to avoid using system/culture defaults, as defaults are not - for dates and : for time in all cultures.

Fixes

twilio#596

Checklist

• I acknowledge that all my contributions will be made under the project's license
• I have made a material change to the repo (functionality, testing, spelling, grammar)
• I have read the Contribution Guidelines and my PR follows them
• I have titled the PR appropriately
• I have updated my branch with the main branch
• I have added tests that prove my fix is effective or that my feature works
  • passing existing date serialization test should be sufficient
• I have added the necessary documentation about the functionality in the appropriate .md file
  • This should not be a breaking change - but has the chance to be if people have written very stupid code to change the produced request object instead of changing cultures.
• I have added inline documentation to the code I modified