Introduce a UserAgent option in the Reddit provider

Author: CrustyJew

src/AspNet.Security.OAuth.Reddit/RedditAuthenticationHandler.cs

@@ -36,6 +36,13 @@ protected override async Task<AuthenticationTicket> CreateTicketAsync([NotNull]
             request.Headers.Accept.Add(new MediaTypeWithQualityHeaderValue("application/json"));
             request.Headers.Authorization = new AuthenticationHeaderValue("bearer", tokens.AccessToken);
 
+            // When a custom user agent is specified in the options, add it to the request headers
+            // to override the default (generic) user agent used by the OAuth2 base middleware.
+            if (!string.IsNullOrEmpty(Options.UserAgent))
+            {
+                request.Headers.UserAgent.ParseAdd(Options.UserAgent);
+            }
+
             var response = await Backchannel.SendAsync(request, HttpCompletionOption.ResponseHeadersRead, Context.RequestAborted);
             if (!response.IsSuccessStatusCode)
             {
@@ -68,7 +75,7 @@ protected override string BuildChallengeUrl(AuthenticationProperties properties,
             var address = base.BuildChallengeUrl(properties, redirectUri);
 
             // Add duration=permanent to the authorization request to get an access token that doesn't expire after 1 hour.
-            // See https://github.com/reddit/reddit/wiki/OAuth2#authorization.
+            // See https://github.com/reddit/reddit/wiki/OAuth2#authorization for more information.
             return QueryHelpers.AddQueryString(address, name: "duration", value: "permanent");
         }
 
@@ -88,6 +95,13 @@ protected override async Task<OAuthTokenResponse> ExchangeCodeAsync([NotNull] st
             request.Headers.Accept.Add(new MediaTypeWithQualityHeaderValue("application/json"));
             request.Headers.Authorization = new AuthenticationHeaderValue("Basic", credentials);
 
+            // When a custom user agent is specified in the options, add it to the request headers
+            // to override the default (generic) user agent used by the OAuth2 base middleware.
+            if (!string.IsNullOrEmpty(Options.UserAgent))
+            {
+                request.Headers.UserAgent.ParseAdd(Options.UserAgent);
+            }
+
             request.Content = new FormUrlEncodedContent(new Dictionary<string, string>
             {
                 ["grant_type"] = "authorization_code",

src/AspNet.Security.OAuth.Reddit/RedditAuthenticationOptions.cs

@@ -28,5 +28,12 @@ public RedditAuthenticationOptions()
 
             Scope.Add("identity");
         }
+
+        /// <summary>
+        /// Gets or sets the user agent string to pass when sending requests to Reddit.
+        /// Setting this option is strongly recommended to prevent request throttling.
+        /// For more information, visit https://github.com/reddit/reddit/wiki/API.
+        /// </summary>
+        public string UserAgent { get; set; }
     }
 }