Merge pull request #645 from whoisj/fix-ghauth

github: conditionally append OTP header

Author: J Wyman

GitHub.Authentication/Src/Authority.cs

@@ -86,7 +86,11 @@ public async Task<AuthenticationResult> AcquireToken(
             };
 
             options.Headers.Accept.Add(new MediaTypeWithQualityHeaderValue(GitHubApiAcceptsHeaderValue));
-            options.Headers.Add(GitHubOptHeader, authenticationCode);
+
+            if (!string.IsNullOrWhiteSpace(authenticationCode))
+            {
+                options.Headers.Add(GitHubOptHeader, authenticationCode);
+            }
 
             // Create the authority Uri.
             var requestUri = targetUri.CreateWith(_authorityUrl);