Issue title: Bug in Authorization Header construction
 Issue body: I am attempting to authenticate to an OAuth2 service. I am using this code:

> [RestSharp.RestClient].Authenticator = new RestSharp.Authenticators.OAuth2.OAuth2AuthorizationRequestHeaderAuthenticator(token, "Bearer");

This way of Authentication attaches an Authorization header, however it is encoded, when it should not be, resulting in a header with the form of
> Authorization: Bearer%20[token]

as it encodes the space in the header.

 Manually attaching an Authorization header using 
> [RestSharp.RestRequest].AddHeader("Authorization", $"Bearer {token}", **false**)

works and resolves the issue, however the OAuth2 authentication procedure should work too.
 Comments: 
 Comment 0: Which version is that?
 Comment 1: 107.0.0-preview.14
 Comment 2: Preview 17 should have this issue fixed.
{'source': 'GithubIssues', 'repo_name_with_owner': 'restsharp/RestSharp', 'issue_id': 1089535872, 'repo_stars': 9117.0}