Implement WithError(err) in exported, fixed doco.

entry_test.go

@@ -10,16 +10,25 @@ import (
 
 func TestEntryWithError(t *testing.T) {
 
+	assert := assert.New(t)
+
+	defer func() {
+		ErrorKey = "error"
+	}()
+
 	err := fmt.Errorf("kaboom at layer %d", 4711)
 
+	assert.Equal(err, WithError(err).Data["error"])
+
 	logger := New()
 	logger.Out = &bytes.Buffer{}
 	entry := NewEntry(logger)
 
-	assert.Equal(t, err, entry.WithError(err).Data["error"])
+	assert.Equal(err, entry.WithError(err).Data["error"])
 
 	ErrorKey = "err"
-	assert.Equal(t, err, entry.WithError(err).Data["err"])
+
+	assert.Equal(err, entry.WithError(err).Data["err"])
 
 }