Allow calling Draft4Support.Enable() multiple times

[laurynasr]

Second call to Draft4Support.Enable() was throwing

  Failed MultipleEnableDoesNotThrow [36 ms]
  Error Message:
   System.ArgumentException : Overriding known specification versions is not supported.
  Stack Trace:
     at Json.Schema.SchemaRegistry.RegisterNewSpecVersion(Uri metaSchemaUri, SpecVersion specVersion)
   at Graeae.Models.SchemaDraft4.Draft4Support.Enable() in D:\src\Graeae\Graeae.Models\SchemaDraft4\Draft4Support.cs:line 181
   at Graeae.Models.Tests.ApiTests.MultipleEnableDoesNotThrow() in D:\src\Graeae\Graeae.Models.Tests\ApiTests.cs:line 10

[gregsdennis]

I'm not sure this is the right direction. Draft4Support.Enable() is a bootstrapping call. It should only ever be called during application setup.

What's your use case?

[laurynasr]

We use it in multiple test fixtures that might be executed all at once or separately by filtering -- and initialization is per fixture, not per project. This is also something that worked in Graeae 0.2

[gregsdennis]

I don't like the idea of setting up production code just to facilitate tests, but I agree that this method should be safe to re-call.

Please add an entry in the RELEASE_NOTES.md file.

[gregsdennis]

remove unnecessary edit, please.