Have prefixFieldClashes respect the JSON FieldMap

Currently, it will fix clashes that don't exist and miss clashes that
do exist due to field remapping.

Fixes #708

formatter.go

@@ -30,16 +30,19 @@ type Formatter interface {
 //
 // It's not exported because it's still using Data in an opinionated way. It's to
 // avoid code duplication between the two default formatters.
-func prefixFieldClashes(data Fields) {
-	if t, ok := data["time"]; ok {
-		data["fields.time"] = t
+func prefixFieldClashes(data Fields, fieldMap FieldMap) {
+	timeKey := fieldMap.resolve(FieldKeyTime)
+	if t, ok := data[timeKey]; ok {
+		data["fields."+timeKey] = t
 	}
 
-	if m, ok := data["msg"]; ok {
-		data["fields.msg"] = m
+	msgKey := fieldMap.resolve(FieldKeyMsg)
+	if m, ok := data[msgKey]; ok {
+		data["fields."+msgKey] = m
 	}
 
-	if l, ok := data["level"]; ok {
-		data["fields.level"] = l
+	levelKey := fieldMap.resolve(FieldKeyLevel)
+	if l, ok := data[levelKey]; ok {
+		data["fields."+levelKey] = l
 	}
 }