Fix uninitialized error detection

Incorporate fix for go-errors#36

Author: scudette

error.go

@@ -91,7 +91,7 @@ func New(e interface{}) *Error {
 // fmt.Errorf("%v"). The skip parameter indicates how far up the stack
 // to start the stacktrace. 0 is from the current call, 1 from its caller, etc.
 func Wrap(e interface{}, skip int) *Error {
-	if e == nil {
+	if IsUninitialized(e) {
 		return nil
 	}
 
@@ -114,6 +114,20 @@ func Wrap(e interface{}, skip int) *Error {
 	}
 }
 
+// IsUninitialized returns true if the error is nil or is zero value
+func IsUninitialized(i interface{}) bool {
+	if i == nil {
+		return true
+	}
+	switch reflect.TypeOf(i).Kind() {
+	case reflect.Struct:
+		return reflect.ValueOf(i).IsZero()
+	case reflect.Ptr, reflect.Map, reflect.Array, reflect.Chan, reflect.Slice:
+		return reflect.ValueOf(i).IsNil()
+	}
+	return false
+}
+
 // WrapPrefix makes an Error from the given value. If that value is already an
 // error then it will be used directly, if not, it will be passed to
 // fmt.Errorf("%v"). The prefix parameter is used to add a prefix to the