Send CORS allowed headers in API response (#114)

Sends Access-Control-Allow-Headers Content-Type and Authorization in the API response headers

PluralKit.API/Startup.cs

@@ -49,7 +49,7 @@ namespace PluralKit.API
             }
 
             //app.UseHttpsRedirection();
-            app.UseCors(opts => opts.AllowAnyMethod().AllowAnyOrigin());
+            app.UseCors(opts => opts.AllowAnyMethod().AllowAnyOrigin().WithHeaders("Content-Type", "Authorization"));
             app.UseMiddleware<TokenAuthService>();
             app.UseMvc();
         }