Cleanup and consolidate check

Author: shana

src/GitHub.Api/SimpleApiClient.cs

@@ -74,7 +74,7 @@ async Task<Repository> GetRepositoryInternal()
             catch (ApiException apiex)
             {
                 if (!HostAddress.IsGitHubDotComUri(OriginalUrl.ToRepositoryUrl()))
-                    isEnterprise = apiex.HttpResponse?.Headers.ContainsKey("X-GitHub-Request-Id");
+                    isEnterprise = apiex.IsGitHubApiException();
             }
             catch {}
             finally

src/GitHub.Exports/ExceptionExtensions.cs

@@ -0,0 +1,15 @@
+using System;
+using Octokit;
+
+namespace GitHub.Extensions
+{
+    public static class ExceptionExtensions
+    {
+        const string GithubHeader = "X-GitHub-Request-Id";
+        public static bool IsGitHubApiException(this Exception ex)
+        {
+            var apiex = ex as ApiException;
+            return apiex?.HttpResponse?.Headers.ContainsKey(GithubHeader) ?? false;
+        }
+    }
+}

src/GitHub.Exports/Services/EnterpriseProbeTask.cs

@@ -41,9 +41,7 @@ public async Task<EnterpriseProbeResult> ProbeAsync(Uri enterpriseBaseUrl)
             var ret = await httpClient
                     .Send(request, CancellationToken.None)
                     .Catch(ex => {
-                        var apiex = ex as ApiException;
-                        if (apiex != null)
-                            success = apiex.HttpResponse?.Headers.ContainsKey("X-GitHub-Request-Id") ?? false;
+                        success = ex.IsGitHubApiException();
                         return null;
                     });