Inconsistent handling of generic type definitions

[CptWesley]

Currenlty the implementation of GetDatabaseType and ToParameterType treat types with generics in different ways:

• GetDatabaseType always strips the concrete generic arguments to retrieve the generic type definition and uses that to try and retrieve the registered type name.
• ToParameterType does not have special handling.

I would expect these two functions to treat generic types in the same way (otherwise it is always required to add 1 concrete and 1 open version to e.g. the NpgsqlTypeMapping implementation).

My proposal would be to first attempt to find a mapping for the type directly, if there is no such definition, retry on the open generic version of it.

This will most likely also resolve this issue: JasperFx/marten#3148

Will create pull request with proposed solution