Add Go Report Card, fix a couple of golint warnings.

README.md

@@ -1,4 +1,4 @@
-Go smtpd [![GoDoc](https://godoc.org/github.com/chrj/smtpd?status.png)](https://godoc.org/github.com/chrj/smtpd)
+Go smtpd [![GoDoc](https://godoc.org/github.com/chrj/smtpd?status.png)](https://godoc.org/github.com/chrj/smtpd) [![Go Report Card](https://goreportcard.com/badge/github.com/chrj/smtpd)](https://goreportcard.com/report/github.com/chrj/smtpd)
 ========
 
 Package smtpd implements an SMTP server in golang.

example_test.go

@@ -1,10 +1,11 @@
 package smtpd_test
 
 import (
-	"github.com/chrj/smtpd"
 	"errors"
 	"net/smtp"
 	"strings"
+
+	"github.com/chrj/smtpd"
 )
 
 func ExampleServer() {

protocol.go

@@ -524,11 +524,10 @@ func (session *session) handleXCLIENT(cmd command) {
 	}
 
 	var (
-		newHeloName          = ""
-		newAddr     net.IP   = nil
-		newTCPPort  uint64   = 0
-		newUsername          = ""
-		newProto    Protocol = ""
+		newHeloName, newUsername string
+		newProto                 Protocol
+		newAddr                  net.IP
+		newTCPPort               uint64
 	)
 
 	for _, item := range cmd.fields[1:] {

smtpd.go

@@ -55,8 +55,11 @@ type Server struct {
 type Protocol string
 
 const (
-	SMTP  Protocol = "SMTP"
-	ESMTP          = "ESMTP"
+	// SMTP
+	SMTP Protocol = "SMTP"
+
+	// Extended SMTP
+	ESMTP = "ESMTP"
 )
 
 // Peer represents the client connecting to the server