Shouldn't default scope seperator (delimeter) be space?

[Kudze]

Hello,

I have been using this library to implement OAuth2 authorization with Canva. During this work, I have noticed an issue, when I request multiple scopes is that in HTTP request they are seperated by comma , by default. However, Canva expects a space. I have also checked The OAuth 2.0 Authorization Framework RFC. There they define that scope delimeter should be a space too.

I am aware that the scopeSeparator may be configured via options. However, shouldn't default value (defined in League\OAuth2\Client\Provider\AbstractProvider::getScopeSeparator()) match with whats defined in RFC?

I am able to submit PR for this. However, it would be a breaking change for some of the library users (that depend on default value to be ,).