allow Add to *replace* an existing template parameter in DynamicParameters, allowing for out/return/etc usage

fix 'could destabilise runtime' error if structs threw an error; also tidied up the locals and made the func return the right type

patch by  matt...@amigarulez.se, Some grids active/Telerik and other edge cases use reflection and other methods to cache members.

Current implementation results in inconsistent between the wrapper object and the underlying dictionary because the DynamicObject members are returned and not the underlying dictionary.

Simple fix basically add an override to FastExpando class and reflection acts consistently.

patch by  matt...@amigarulez.se, Some grids active/Telerik and other edge cases use reflection and other methods to cache members.

Current implementation results in inconsistent between the wrapper object and the underlying dictionary because the DynamicObject members are returned and not the underlying dictionary.

Simple fix basically add an override to FastExpando class and reflection acts consistently.