Fix assigning to Nullable Enum

[KurtGokhan]

Summary:

• added test for nullable enum and struct assignment
• fix nullable enum assignment
• microoptimizations in DefaultTypeConverter (I am not sure about its benefits)

Also #153 and #157 are not reproducible anymore.

[sebastienros]

I think typeof expression are resolved at compile time. Also private member should be prefixed with _, or in upper case for constants. Merging as these are minor.