Code Fix for NUnit2010 on Ref Structs Creates CS0306

[manfred-brands]

Relates to #475 where the detected code was actual == expected.
However NUnit2010 is also raised for actual.Equals(expected)
This was addressed partly in #477 but only if the argument was a ref readonly struct not if the instance was one.

[Test]
public void TestMethod()
{
    ReadOnlySpan<char> actual = GetPart();
    Assert.That(actual.Equals(""Text""), Is.True);
}

ReadOnlySpan<char> GetPart() => ""Some Text"".AsSpan(4);

Here the method ReadOnlySpan<char>.Equals(string) is called.

The CodeFix converts this into: Assert.That(actual, Is.EqualTo(expected));
which raises: error CS0306: The type 'ReadOnlySpan<char>' may not be used as a type argument