Change "BASIC" to "Basic"

After further discussion and research in manfredsteyer#368 I noticed that even
though RFC 7617 seems to state that:

> Note that both scheme and parameter names are matched
> case-insensitively.

All the examples use "Basic" instead of "BASIC". There is one very
notable framework that checks case *sensitively* for "Basic". Given
that the header was incorrect anyways, we can be sure that changing
this casing won't further break anything, and work around issues for
server side frameworks like Spring.

See: https://tools.ietf.org/html/rfc7617
See: https://github.com/spring-projects/spring-security/blob/5.0.6.RELEASE/web/src/main/java/org/springframework/security/web/authentication/www/BasicAuthenticationFilter.java#L157

Fixes manfredsteyer#368

Author: jeroenheijmans

projects/lib/src/oauth-service.ts

@@ -639,7 +639,7 @@ export class OAuthService extends AuthConfig {
                 const header = btoa(`${this.clientId}:${this.dummyClientSecret}`);
                 headers = headers.set(
                     'Authorization',
-                    'BASIC ' + header);
+                    'Basic ' + header);
             }
 
             if (!this.useHttpBasicAuthForPasswordFlow) {