GetDefinitionsProductType returning error due to incorrect serialization of Enum

[Chu2P]

When calling the GetDefinitionsProductType endpoint with the LISTINGPRODUCTONLY requirement, we get

AmazonInvalidInputException: 'Request has missing or invalid parameters and cannot be parsed.'

This is due to the Conversion of parameters in ParameterBased.cs (ln 55) not converting the enum to its EnumMember value, therefore losing the required underscore characters