json_formatter: always cast errors to strings

Fixes #137

json_formatter.go

diff --git a/json_formatter.go b/json_formatter.go
index b09227c..0e38a61 100644
--- a/json_formatter.go
+++ b/json_formatter.go
@@ -11,7 +11,13 @@
 func (f *JSONFormatter) Format(entry *Entry) ([]byte, error) {
 	data := make(Fields, len(entry.Data)+3)
 	for k, v := range entry.Data {
-		data[k] = v
+		// 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 {
+			data[k] = v
+		}
 	}
 	prefixFieldClashes(data)
 	data["time"] = entry.Time.Format(time.RFC3339)

json_formatter_test.go

diff --git a/json_formatter_test.go b/json_formatter_test.go
new file mode 100644
index 0000000..de19bc1
--- /dev/null
+++ b/json_formatter_test.go
@@ -0,0 +1,46 @@
+package logrus
+
+import (
+	"encoding/json"
+	"errors"
+
+	"testing"
+)
+
+func TestErrorNotLost(t *testing.T) {
+	formatter := &JSONFormatter{}
+
+	b, err := formatter.Format(WithField("error", errors.New("wild walrus")))
+	if err != nil {
+		t.Fatal("Unable to format entry: ", err)
+	}
+
+	entry := make(map[string]interface{})
+	err = json.Unmarshal(b, &entry)
+	if err != nil {
+		t.Fatal("Unable to unmarshal formatted entry: ", err)
+	}
+
+	if entry["error"] != "wild walrus" {
+		t.Fatal("Error field not set")
+	}
+}
+
+func TestErrorNotLostOnFieldNotNamedError(t *testing.T) {
+	formatter := &JSONFormatter{}
+
+	b, err := formatter.Format(WithField("omg", errors.New("wild walrus")))
+	if err != nil {
+		t.Fatal("Unable to format entry: ", err)
+	}
+
+	entry := make(map[string]interface{})
+	err = json.Unmarshal(b, &entry)
+	if err != nil {
+		t.Fatal("Unable to unmarshal formatted entry: ", err)
+	}
+
+	if entry["omg"] != "wild walrus" {
+		t.Fatal("Error field not set")
+	}
+}