[JSON] Use type-switch for error field

json_formatter.go

diff --git a/json_formatter.go b/json_formatter.go
index 0e38a61..5c4c44b 100644
--- a/json_formatter.go
+++ b/json_formatter.go
@@ -11,11 +11,12 @@
 func (f *JSONFormatter) Format(entry *Entry) ([]byte, error) {
 	data := make(Fields, len(entry.Data)+3)
 	for k, v := range entry.Data {
-		// Otherwise errors are ignored by `encoding/json`
-		// https://github.com/Sirupsen/logrus/issues/137
-		if err, ok := v.(error); ok {
-			data[k] = err.Error()
-		} else {
+		switch v := v.(type) {
+		case error:
+			// Otherwise errors are ignored by `encoding/json`
+			// https://github.com/Sirupsen/logrus/issues/137
+			data[k] = v.Error()
+		default:
 			data[k] = v
 		}
 	}