crypto/x509: use Certificate.Equals and CertPool.contains.

By using these utility functions, the code can be made a little shorter.
Thanks to Omar Shafie for pointing this out in
https://golang.org/cl/27393/.

Change-Id: I33fd97cf7d60a31d0844ec16c12bba530dcc6f6d
Reviewed-on: https://go-review.googlesource.com/32120
Run-TryBot: Adam Langley <agl@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/crypto/x509/cert_pool.go

@@ -4,10 +4,7 @@
 
 package x509
 
-import (
-	"bytes"
-	"encoding/pem"
-)
+import "encoding/pem"
 
 // CertPool is a set of certificates.
 type CertPool struct {
@@ -67,7 +64,7 @@ func (s *CertPool) contains(cert *Certificate) bool {
 
 	candidates := s.byName[string(cert.RawSubject)]
 	for _, c := range candidates {
-		if bytes.Equal(cert.Raw, s.certs[c].Raw) {
+		if s.certs[c].Equal(cert) {
 			return true
 		}
 	}
@@ -82,10 +79,8 @@ func (s *CertPool) AddCert(cert *Certificate) {
 	}
 
 	// Check that the certificate isn't being added twice.
-	for _, c := range s.certs {
-		if c.Equal(cert) {
-			return
-		}
+	if s.contains(cert) {
+		return
 	}
 
 	n := len(s.certs)