Change "state" from string to enum in Migration Response.

Octokit.Tests/Models/MigrationTests.cs

@@ -113,7 +113,7 @@ namespace Octokit.Tests.Models
         private static readonly Migration migration = new Migration(
             id: 79,
             guid: "0b989ba4-242f-11e5-81e1-c7b6966d2516",
-            state: "exported",
+            state: Migration.MigrationState.Exported, 
             lockRepositories: true,
             excludeAttachments: false,
             url: "https://api.github.com/orgs/octo-org/migrations/79",
@@ -135,6 +135,7 @@ namespace Octokit.Tests.Models
             Assert.Equal("0b989ba4-242f-11e5-81e1-c7b6966d2516", _migration.Guid);
             Assert.Equal(1, _migration.Repositories.Count);
             Assert.Equal(1296269, _migration.Repositories[0].Id);
+            Assert.Equal(Migration.MigrationState.Pending, _migration.State);
         }
     }
 

Octokit/Models/Response/Migration.cs

@@ -23,7 +23,7 @@ namespace Octokit
         public Migration(
             int id,
             string guid,
-            string state,
+            MigrationState state,
             bool lockRepositories,
             bool excludeAttachments,
             string url,
@@ -55,7 +55,7 @@ namespace Octokit
         /// <summary>
         /// The state of migration. Can be one of pending, exporting, exported and failed.
         /// </summary>
-        public string State { get; private set; }
+        public MigrationState State { get; private set; }
 
         /// <summary>
         /// Whether to lock repositories.
@@ -94,5 +94,34 @@ namespace Octokit
                 return string.Format(CultureInfo.InvariantCulture, "Guid: {0}", Guid);
             }
         }
+
+        /// <summary>
+        /// State of a migration.
+        /// </summary>
+        /// <remarks>
+        /// See: https://developer.github.com/v3/migration/migrations/#get-the-status-of-a-migration
+        /// </remarks>
+        public enum MigrationState
+        {
+            /// <summary>
+            /// The migration hasn't started yet.
+            /// </summary>
+            Pending,
+
+            /// <summary>
+            /// The migration is in progress.
+            /// </summary>
+            Exporting,
+
+            /// <summary>
+            /// The migration finished successfully.
+            /// </summary>
+            Exported,
+
+            /// <summary>
+            /// The migration failed.
+            /// </summary>
+            Failed
+        }
     }
 }