Fix Cannot Attach Entity to DataServiceContext if Entity Key is an Enum

[WanjohiSammy]

Issues

This pull request fixes #3080.

Description

This change is to add EdmValueKind.Enum case in ToClrValue in order to be able to get the Value property of the IEdmEnumValue. This means that when the edmValue is of type EdmValueKind.Enum, the method will return the underlying value of the enum instead of throwing the below exception:

Microsoft.OData.ODataException: A value with primitive kind 'Enum' cannot be converted into a primitive object value.
   at Microsoft.OData.Client.EdmValueUtils.ToClrValue(IEdmPrimitiveValue edmValue)
   at Microsoft.OData.Client.DataServiceUrlKeyDelimiter.<>c__DisplayClass9_0`1.<AppendKeyExpression>b__0(T p)
   at Microsoft.OData.Client.KeySerializer.GetKeyValueAsString[TProperty](Func`2 getPropertyValue, TProperty property, LiteralFormatter literalFormatter)
   at Microsoft.OData.Client.KeySerializer.AppendKeyWithParentheses[TProperty](StringBuilder builder, ICollection`1 keyProperties, Func`2 getPropertyName, Func`2 getPropertyValue)
   at Microsoft.OData.Client.DataServiceUrlKeyDelimiter.AppendKeyExpression(IEdmStructuredValue entity, StringBuilder builder)
   at Microsoft.OData.Client.ConventionalODataEntityMetadataBuilder.ConventionalODataUriBuilder.BuildEntityInstanceUri(Uri baseUri, IEdmStructuredValue entityInstance)
   at Microsoft.OData.Client.DataServiceContext.AttachTo(String entitySetName, Object entity, String etag)

Checklist (Uncheck if it is not completed)

• Test cases added
• Build and test with one-click build and test script passed