Memory Leak on MethodBinding for generic method #691

Environment

• Pythonnet version: 2.3.0
• Python version: 3.6-32
• Operating System: Windows 10

Details

• Getting a MethodBinding for a generic method results in the reference count being too high, so the tp_dealloc method on the MethodBinding never gets called, even when the MethodBinding goes out of scope.

namespace PlainOldNamespace
{
    public class PlainOldClass
    {
        public void NonGenericMethod() { }

        public void GenericMethod<T>() { }
    }
}

import clr, sys
clr.AddReference("PlainOldAssembly")
from PlainOldNamespace import PlainOldClass
poc = PlainOldClass()
sys.getrefcount(poc.NonGenericMethod)
# 1
sys.getrefcount(poc.GenericMethod[clr.System.Double])
# 2 - should be 1

I think this could be down to a bug in MethodBinding.cs. This line looks suspicious. Why should the reference count get incremented here?
https://github.com/pythonnet/pythonnet/blob/master/src/runtime/methodbinding.cs#L59