Add StringEnum to handle unknown enum values returned from API (#1595)

* Added StringEnum<TEnum>

* Added tests

* Make sure the serializer can work with StringEnum

* Use StringEnum for EventInfo.Event

* Add convention test to assert that all Response models use StringEnum<> to wrap enum properties

* Add Stringnum<> to all response types failing convention test

* Handle StringEnum to Enum conversion when Issue response model populates IssueUpdate request model

* Fix unit test

* Refactor SimpleJsonSerializer to expose the DeserializeEnum strategy so it can be used in StringEnum class

* Need to expose/use SerializeEnum functionality too, so we use the correct string representation of enum values that have custom properties (eg ReactionType Plus1 to "+1")

* fix unit tests, since the string is now the "correct" upstream api value

* Add a couple of tests for the Enum serialize/deserialize when underscores, hyphens and custom property attributes are present

* Compare parsed values for equality

* add convention test to ensure enum members all have Parameter property set

* update test to cover implicit conversions too

* this test should work but fails at the moment due to magic hyphen removal in deserializer causing a one way trip from utf-8 to EncodingType.Utf8 with no way to get back

* (unsuccesfully) expand event info test to try to catch more cases of unknown event types

* fix broken integration test while im here

* Fixed build errors after .NET Core merge

* Value -> StringValue, ParsedValue -> Value

* Don't allow StringValue to be null

* Ignore enums not used in request/response models

* Added ParameterAttribute to almost all enum values

* Ignore Language enum

* Fix failing tests

* Fix milestone sort parameter and tests

* whitespace

* fix milestone unit tests

* Fix StringEnum.Equals ... This could've been embarrassing!

* Change SimpleJsonSerializer Enum handling to only use `[Parameter()]` attributes (no more magic removal of hyphen/underscores from strings)

* Tidy up this integration test while im here

* Only test request/response enums in convention test

* Keep skipping Language

* Remove unused method

* Remove excluded enum types

* Removed unnecessary ParameterAttributes

* Remove unused enum

* Add StringEnum test for string-comparison of two invalid values

* Bring back IssueCommentSort and use it in IssueCommentRequest

This reverts commit 38a4a291d1476ef8c992fe0f76956974b6f32a49.

* Use assembly instead of namespace for Octokit check

* Add failing test to reproduce the issue where only the first enum paramter/value was added to the cache

* Fix deserializer enum cache to include all enum members rather than only the first member encountered

* Use a static SimpleJsonSerializer in StringEnum

* Remove serializer instance in StringEnum

* Add some documentation on StringEnum<TEnum>

* Fix parameter value to resolve failing integration test

Octokit/Models/Request/MergePullRequest.cs

@@ -1,6 +1,7 @@
 using System;
 using System.Diagnostics;
 using System.Globalization;
+using Octokit.Internal;
 
 namespace Octokit
 {
@@ -56,16 +57,19 @@ namespace Octokit
         /// <summary>
         /// Create a merge commit
         /// </summary>
+        [Parameter(Value = "merge")]
         Merge,
 
         /// <summary>
         /// Squash and merge
         /// </summary>
+        [Parameter(Value = "squash")]
         Squash,
 
         /// <summary>
         /// Rebase and merge
         /// </summary>
+        [Parameter(Value = "rebase")]
         Rebase
     }
 }