fix golint issues in core/codec (#473)

core/codec/hmac.go

@@ -7,12 +7,14 @@ import (
 	"io"
 )
 
+// Hmac returns HMAC bytes for body with the given key.
 func Hmac(key []byte, body string) []byte {
 	h := hmac.New(sha256.New, key)
 	io.WriteString(h, body)
 	return h.Sum(nil)
 }
 
+// HmacBase64 returns the base64 encoded string of HMAC for body with the given key.
 func HmacBase64(key []byte, body string) string {
 	return base64.StdEncoding.EncodeToString(Hmac(key, body))
 }