Polyfill.IsNullable(this MemberInfo info) extension conflicts with my own

[hazzik]

Describe the bug

I had my own extension:

public static bool IsNullable(this Type type)
{ 
  return type.IsGenericType && type.GetGenericTypeDefinition() == typeof(Nullable<>);
}

And Polyfill.IsNullable(this MemberInfo info) extensions takes precedence over mine. I suppose it happens because Polyfill is in root namespace and has more generic type.

This leads to the errors at runtime:

----> System.ArgumentException : Unsupported type:System.RuntimeType
  Stack Trace:
     at FluentNHibernate.Cfg.FluentConfiguration.BuildSessionFactory() in /_/src/FluentNHibernate/Cfg/FluentConfiguration.cs:line 229
   at FluentNHibernate.SessionSource..ctor(FluentConfiguration config) in /_/src/FluentNHibernate/SessionSource.cs:line 40
   at FluentNHibernate.Testing.Testing.with_fluent_configuration.build_session_source() in /_/src/FluentNHibernate.Testing/Testing/SessionSourceConfigurationTester.cs:line 68
   at FluentNHibernate.Testing.Testing.when_using_a_session_source_and_schema.establish_context() in /_/src/FluentNHibernate.Testing/Testing/SessionSourceConfigurationTester.cs:line 38
   at FluentNHibernate.Testing.Specification.Setup() in /_/src/FluentNHibernate.Testing/Specification.cs:line 16
--FluentConfigurationException
   at FluentNHibernate.Cfg.FluentConfiguration.BuildConfiguration() in /_/src/FluentNHibernate/Cfg/FluentConfiguration.cs:line 258
   at FluentNHibernate.Cfg.FluentConfiguration.BuildSessionFactory() in /_/src/FluentNHibernate/Cfg/FluentConfiguration.cs:line 226
--ArgumentException
   at Polyfill.GetNullabilityInfo(MemberInfo info) in /_1/polyfill/3.0.0/contentFiles/cs/netstandard2.0/Polyfill/Nullability/NullabilityInfoExtensions.cs:line 39
   at FluentNHibernate.Mapping.PropertyPart.FluentNHibernate.Mapping.Providers.IPropertyMappingProvider.GetPropertyMapping() in /_/src/FluentNHibernate/Mapping/PropertyPart.cs:line 327
   at FluentNHibernate.Mapping.ClassMap`1.FluentNHibernate.IMappingProvider.GetClassMapping() in /_/src/FluentNHibernate/Mapping/ClassMap.cs:line 611
   at FluentNHibernate.PersistenceModel.BuildSeparateMappings(Action`1 add) in /_/src/FluentNHibernate/PersistenceModel.cs:line 180
   at FluentNHibernate.PersistenceModel.BuildMappings() in /_/src/FluentNHibernate/PersistenceModel.cs:line 167
   at FluentNHibernate.PersistenceModel.EnsureMappingsBuilt() in /_/src/FluentNHibernate/PersistenceModel.cs:line 220
   at FluentNHibernate.PersistenceModel.Configure(Configuration cfg) in /_/src/FluentNHibernate/PersistenceModel.cs:line 278
   at FluentNHibernate.Cfg.FluentConfiguration.BuildConfiguration() in /_/src/FluentNHibernate/Cfg/FluentConfiguration.cs:line 250

Minimal Repro

Not so minimal: nhibernate/fluent-nhibernate#657

Submit a PR that fixes the bug

N/A

[SimonCropp]

i will change that to an op in api. new version eta tomorrow