Should throw on unused arg matchers

[dtchepak]

In this example the problem occurs because int implicitly casts to double, but NSubstitute doesn't pick up the argument matcher as the types diff (changing that opens a can of worms IIRC). I think that that when Returns runs the "any int" matcher is discarded; instead it should throw as the call has not been specified correctly.

        public interface IFoo {
            int Blah(double s);
        }

        [Test]
        public void NSubRepro()
        {
            var foo = NSubstitute.Substitute.For<IFoo>();
            foo.Blah(Arg.Any<int>()).Returns(42);
            Assert.AreEqual(42, foo.Blah(10)); 
            /* FAILS: 
                  Expected: 42
                  But was:  0 */
        }