[FIXED] assignment copies lock value for crypto/tls.Config

Running `go vet ./...` with `go 1.7.3` would report the following:

```
nats.go:769: assignment copies lock value to tlsCopy: crypto/tls.Config contains sync.Once contains sync.Mutex
```

Add a “clone” function while waiting for this to be addressed
by the language itself (https://go-review.googlesource.com/#/c/28075/)

.travis.yml

@@ -1,8 +1,8 @@
 language: go
 sudo: false
 go:
-- 1.5
-- 1.6
+- 1.6.3
+- 1.7.3
 install:
 - go get -t ./...
 - go get github.com/nats-io/gnatsd

nats.go

@@ -23,6 +23,7 @@ import (
 	"sync/atomic"
 	"time"
 
+	"github.com/nats-io/nats/util"
 	"github.com/nats-io/nuid"
 )
 
@@ -766,13 +767,13 @@ func (nc *Conn) makeTLSConn() {
 	// default to InsecureSkipVerify.
 	// TODO(dlc) - We should make the more secure version the default.
 	if nc.Opts.TLSConfig != nil {
-		tlsCopy := *nc.Opts.TLSConfig
+		tlsCopy := util.CloneTLSConfig(nc.Opts.TLSConfig)
 		// If its blank we will override it with the current host
 		if tlsCopy.ServerName == _EMPTY_ {
 			h, _, _ := net.SplitHostPort(nc.url.Host)
 			tlsCopy.ServerName = h
 		}
-		nc.conn = tls.Client(nc.conn, &tlsCopy)
+		nc.conn = tls.Client(nc.conn, tlsCopy)
 	} else {
 		nc.conn = tls.Client(nc.conn, &tls.Config{InsecureSkipVerify: true})
 	}

util/tls.go

@@ -0,0 +1,37 @@
+// Copyright 2016 Apcera Inc. All rights reserved.
+// +build go1.7
+
+package util
+
+import (
+	"crypto/tls"
+)
+
+// CloneTLSConfig returns a copy of c. Only the exported fields are copied.
+// This is temporary, until this is provided by the language.
+// https://go-review.googlesource.com/#/c/28075/
+func CloneTLSConfig(c *tls.Config) *tls.Config {
+	return &tls.Config{
+		Rand:                        c.Rand,
+		Time:                        c.Time,
+		Certificates:                c.Certificates,
+		NameToCertificate:           c.NameToCertificate,
+		GetCertificate:              c.GetCertificate,
+		RootCAs:                     c.RootCAs,
+		NextProtos:                  c.NextProtos,
+		ServerName:                  c.ServerName,
+		ClientAuth:                  c.ClientAuth,
+		ClientCAs:                   c.ClientCAs,
+		InsecureSkipVerify:          c.InsecureSkipVerify,
+		CipherSuites:                c.CipherSuites,
+		PreferServerCipherSuites:    c.PreferServerCipherSuites,
+		SessionTicketsDisabled:      c.SessionTicketsDisabled,
+		SessionTicketKey:            c.SessionTicketKey,
+		ClientSessionCache:          c.ClientSessionCache,
+		MinVersion:                  c.MinVersion,
+		MaxVersion:                  c.MaxVersion,
+		CurvePreferences:            c.CurvePreferences,
+		DynamicRecordSizingDisabled: c.DynamicRecordSizingDisabled,
+		Renegotiation:               c.Renegotiation,
+	}
+}

util/tls_pre17.go

@@ -0,0 +1,35 @@
+// Copyright 2016 Apcera Inc. All rights reserved.
+// +build go1.5,!go1.7
+
+package util
+
+import (
+	"crypto/tls"
+)
+
+// CloneTLSConfig returns a copy of c. Only the exported fields are copied.
+// This is temporary, until this is provided by the language.
+// https://go-review.googlesource.com/#/c/28075/
+func CloneTLSConfig(c *tls.Config) *tls.Config {
+	return &tls.Config{
+		Rand:                     c.Rand,
+		Time:                     c.Time,
+		Certificates:             c.Certificates,
+		NameToCertificate:        c.NameToCertificate,
+		GetCertificate:           c.GetCertificate,
+		RootCAs:                  c.RootCAs,
+		NextProtos:               c.NextProtos,
+		ServerName:               c.ServerName,
+		ClientAuth:               c.ClientAuth,
+		ClientCAs:                c.ClientCAs,
+		InsecureSkipVerify:       c.InsecureSkipVerify,
+		CipherSuites:             c.CipherSuites,
+		PreferServerCipherSuites: c.PreferServerCipherSuites,
+		SessionTicketsDisabled:   c.SessionTicketsDisabled,
+		SessionTicketKey:         c.SessionTicketKey,
+		ClientSessionCache:       c.ClientSessionCache,
+		MinVersion:               c.MinVersion,
+		MaxVersion:               c.MaxVersion,
+		CurvePreferences:         c.CurvePreferences,
+	}
+}