add logstash formatter test

formatters/logstash/logstash_formatter_test.go

diff --git a/formatters/logstash/logstash_formatter_test.go b/formatters/logstash/logstash_formatter_test.go
new file mode 100644
index 0000000..7e48201
--- /dev/null
+++ b/formatters/logstash/logstash_formatter_test.go
@@ -0,0 +1,52 @@
+package logrus_logstash
+
+import (
+	"bytes"
+	"encoding/json"
+	"github.com/Sirupsen/logrus"
+	"github.com/stretchr/testify/assert"
+	"testing"
+)
+
+func TestLogstashFormatter(t *testing.T) {
+	assert := assert.New(t)
+
+	lf := LogstashFormatter{Type: "abc"}
+
+	fields := logrus.Fields{
+		"message": "def",
+		"level":   "ijk",
+		"type":    "lmn",
+		"one":     1,
+		"pi":      3.14,
+		"bool":    true,
+	}
+
+	entry := logrus.WithFields(fields)
+	entry.Message = "msg"
+	entry.Level = logrus.InfoLevel
+
+	b, _ := lf.Format(entry)
+
+	var data map[string]interface{}
+	dec := json.NewDecoder(bytes.NewReader(b))
+	dec.UseNumber()
+	dec.Decode(&data)
+
+	// base fields
+	assert.Equal(json.Number("1"), data["@version"])
+	assert.NotEmpty(data["@timestamp"])
+	assert.Equal("abc", data["type"])
+	assert.Equal("msg", data["message"])
+	assert.Equal("info", data["level"])
+
+	// substituted fields
+	assert.Equal("def", data["fields.message"])
+	assert.Equal("ijk", data["fields.level"])
+	assert.Equal("lmn", data["fields.type"])
+
+	// formats
+	assert.Equal(json.Number("1"), data["one"])
+	assert.Equal(json.Number("3.14"), data["pi"])
+	assert.Equal(true, data["bool"])
+}