An odd C# behaviour in our editor.

Steps to reproduce:
- Create two functions in a file like:
private void TestFunc(object value, Action action = null)
and
private void TestFunc(object[] value, Action action = null)

- Unity logs the error:
Assets/TestScript.cs(18,10): error CS0121: The call is ambiguous between the following methods or properties: 'TestScript.TestFunc(object, System.Action)' and 'TestScript.TestFunc(object[], System.Action)'

- When getting rid of the default valued paratemers, it doesn't show the error. Example"
private void TestFunc(object value)
and
private void TestFunc(object[] value)

Tested on Unity 5.3.5f1, 5.4.0b24, and 5.2.5f1; using Monodevelop 5.9.6 on Mac.
This is valid C# code.