fix empty string decoding issue

Author: bosong

numbers.go

@@ -36,7 +36,7 @@ type StringOrNumber string
 
 // UnmarshalJSON implements json.Unmarshaler
 func (n *StringOrNumber) UnmarshalJSON(data []byte) error {
-	if len(data) > 2 && data[0] == '"' && data[len(data)-1] == '"' {
+	if len(data) >= 2 && data[0] == '"' && data[len(data)-1] == '"' {
 		var v string
 		if err := json.Unmarshal(data, &v); err != nil {
 			return err

numbers_test.go

@@ -51,4 +51,10 @@ var _ = Describe("StringOrNumber", func() {
 		Expect(string(bin)).To(Equal(`"33"`))
 	})
 
+	It("should decode empty strings", func() {
+		var n StringOrNumber
+		Expect(json.Unmarshal([]byte(`""`), &n)).To(Succeed())
+		Expect(n).To(Equal(StringOrNumber("")))
+	})
+
 })