Make NotSubset actually fail the test on nil subset

The assertion function was simply returning false, which doesn't actually fail a test.
An example test that should have failed but doesn't:

  func TestNotSubset(t *testing.T) {
      assert.NotSubset(t, []string{"x"}, nil)
  }

assert/assertions.go

@@ -693,7 +693,7 @@ func Subset(t TestingT, list, subset interface{}, msgAndArgs ...interface{}) (ok
 // Returns whether the assertion was successful (true) or not (false).
 func NotSubset(t TestingT, list, subset interface{}, msgAndArgs ...interface{}) (ok bool) {
 	if subset == nil {
-		return false // we consider nil to be equal to the nil set
+		return Fail(t, fmt.Sprintf("nil is the empty set which is a subset of every set"), msgAndArgs...)
 	}
 
 	subsetValue := reflect.ValueOf(subset)

assert/assertions_test.go

@@ -553,6 +553,14 @@ func TestNotSubset(t *testing.T) {
 	}
 }
 
+func TestNotSubsetNil(t *testing.T) {
+	mockT := new(testing.T)
+	NotSubset(mockT, []string{"foo"}, nil)
+	if !mockT.Failed() {
+		t.Error("NotSubset on nil set should have failed the test")
+	}
+}
+
 func Test_includeElement(t *testing.T) {
 
 	list1 := []string{"Foo", "Bar"}