Fix NotNil assertion for typed nil values

The NotNil assertion had an error in its handling of typed nil values.
This change makes use of the helper function isNil (used by the Nil
assertion).  The helper function has correct handling of typed nil
values and when negated provides the expected semantics for
`assert.NotNil(t, x)`.

    if x == nil {
        assert.Fail(t, "is nil", x)
    }

assert/assertions.go

@@ -287,24 +287,10 @@ func Exactly(t TestingT, expected, actual interface{}, msgAndArgs ...interface{}
 //
 // Returns whether the assertion was successful (true) or not (false).
 func NotNil(t TestingT, object interface{}, msgAndArgs ...interface{}) bool {
-
-	success := true
-
-	if object == nil {
-		success = false
-	} else {
-		value := reflect.ValueOf(object)
-		kind := value.Kind()
-		if kind >= reflect.Chan && kind <= reflect.Slice && value.IsNil() {
-			success = false
-		}
+	if !isNil(object) {
+		return true
 	}
-
-	if !success {
-		Fail(t, "Expected value not to be nil.", msgAndArgs...)
-	}
-
-	return success
+	return Fail(t, "Expected value not to be nil.", msgAndArgs...)
 }
 
 // isNil checks if a specified object is nil or not, without Failing.

assert/assertions_test.go

@@ -11,7 +11,7 @@ import (
 )
 
 var (
-	i      interface{}
+	i     interface{}
 	zeros = []interface{}{
 		false,
 		byte(0),
@@ -198,6 +198,9 @@ func TestNotNil(t *testing.T) {
 	if NotNil(mockT, nil) {
 		t.Error("NotNil should return false: object is nil")
 	}
+	if NotNil(mockT, (*struct{})(nil)) {
+		t.Error("NotNil should return false: object is (*struct{})(nil)")
+	}
 
 }