fix non comparable error check

Author: ysmood

assertions.go

@@ -301,7 +301,7 @@ func (as Assertions) Is(x, y interface{}) {
 
 	if ae, ok := x.(error); ok {
 		if be, ok := y.(error); ok {
-			if ae == be {
+			if utils.SmartCompare(ae, be) == 0 {
 				return
 			}
 

assertions_test.go

@@ -320,3 +320,15 @@ func TestCustomAssertionError(t *testing.T) {
 	g.Eq(1, 2)
 	m.check("custom eq")
 }
+
+type rawErr []byte
+
+func (r rawErr) Error() string {
+	return string(r)
+}
+
+func TestIsRawErr(t *testing.T) {
+	g := got.T(t)
+
+	g.Is(rawErr("ok"), rawErr("ok"))
+}