refs #550 - fix random enum failure in csharp

src/main/java/io/swagger/codegen/v3/generators/dotnet/AbstractCSharpCodegen.java

@@ -435,7 +435,7 @@ private void postProcessEnumRefs(final Map<String, Object> models) {
                         // This is different in C# than most other generators, because enums in C# are compiled to integral types,
                         // while enums in many other languages are true objects.
                         CodegenModel refModel = enumRefs.get(var.datatype);
-                        var.allowableValues = refModel.allowableValues;
+                        var.allowableValues = new HashMap<>(refModel.allowableValues);
                         updateCodegenPropertyEnum(var);
 
                         // We do these after updateCodegenPropertyEnum to avoid generalities that don't mesh with C#.