Use space character as the separator for user-agent header instead of comma

[mungojam]

Description

This fixes a signing error that occurs with multiple user agent parts, e.g. Mozilla/5.0 Gecko/geckotrail

• Fixes #1155

Checklist

• [x] My code follows the style guidelines of this project
• [x] I have performed a self-review of my own code
• [x] I have commented my code, particularly in hard-to-understand areas
• [x] I have made corresponding changes to the documentation
• [x] I have added tests that prove my fix is effective or that my feature works

[FantasticFiasco]

These changes looks awesome, added tests and everything!

I'll merge the pr right now, and then tomorrow I'll include it in a new release. Thank you so much for the contribution!

[mungojam]

Thanks for the kind words 🙂, always happy to contribute to such a useful and unique library