Adds missing OAuth login request parameters (#1919)

* Adds support for 'login' parameter
* Adds support for 'allow_signup' parameter

Closes #1918

Octokit.Tests/Clients/OauthClientTests.cs

@@ -44,8 +44,10 @@ public class OauthClientTests
             var request = new OauthLoginRequest("secret")
             {
                 RedirectUri = new Uri("https://example.com/foo?foo=bar"),
+                Login = "johnDOE",
                 Scopes = { "foo", "bar" },
-                State = "canARY"
+                State = "canARY",
+                AllowSignup = false
             };
             var connection = Substitute.For<IConnection>();
             connection.BaseAddress.Returns(new Uri("https://api.github.com"));
@@ -54,7 +56,7 @@ public class OauthClientTests
             var result = client.GetGitHubLoginUrl(request);
 
             Assert.Equal("/login/oauth/authorize", result.AbsolutePath);
-            Assert.Equal("?client_id=secret&redirect_uri=https%3A%2F%2Fexample.com%2Ffoo%3Ffoo%3Dbar&scope=foo%2Cbar&state=canARY", result.Query);
+            Assert.Equal("?client_id=secret&redirect_uri=https%3A%2F%2Fexample.com%2Ffoo%3Ffoo%3Dbar&login=johnDOE&scope=foo%2Cbar&state=canARY&allow_signup=false", result.Query);
         }
     }
 

Octokit/Models/Request/OauthLoginRequest.cs

@@ -40,6 +40,12 @@ namespace Octokit
         [Parameter(Key = "redirect_uri")]
         public Uri RedirectUri { get; set; }
 
+        /// <summary>
+        /// Suggests a specific account to use for signing in and authorizing the app.
+        /// </summary>
+        [Parameter(Key = "login")]
+        public string Login { get; set; }
+
         /// <summary>
         /// A set of scopes to request. If not provided, scope defaults to an empty list of scopes for users that don’t
         /// have a valid token for the app. For users who do already have a valid token for the app, the user won’t be
@@ -60,6 +66,13 @@ namespace Octokit
         [Parameter(Key = "state")]
         public string State { get; set; }
 
+        /// <summary>
+        /// Whether or not unauthenticated users will be offered an option to sign up for GitHub during the OAuth flow.
+        /// The default is <c>true</c>. Use <c>false</c> in the case that a policy prohibits signups.
+        /// </summary>
+        [Parameter(Key = "allow_signup")]
+        public bool? AllowSignup { get; set; }
+
         internal string DebuggerDisplay
         {
             get