fix race conditions on entry

closes #1046

entry.go

@@ -113,6 +113,8 @@ func (entry *Entry) WithField(key string, value interface{}) *Entry {
 
 // Add a map of fields to the Entry.
 func (entry *Entry) WithFields(fields Fields) *Entry {
+	entry.Logger.mu.Lock()
+	defer entry.Logger.mu.Unlock()
 	data := make(Fields, len(entry.Data)+len(fields))
 	for k, v := range entry.Data {
 		data[k] = v