Add base64 encoded MD5 output for Hash Reader (#5315)

- Use it to send the Content-MD5 header correctly encoded to S3
  Gateway

- Fixes a bug in PutObject (including anonymous PutObject) and
  PutObjectPart with S3 Gateway found when testing with Mint.

pkg/hash/reader.go

@@ -20,6 +20,7 @@ import (
 	"bytes"
 	"crypto/md5"
 	"crypto/sha256"
+	"encoding/base64"
 	"encoding/hex"
 	"errors"
 	"hash"
@@ -117,6 +118,11 @@ func (r *Reader) MD5HexString() string {
 	return hex.EncodeToString(r.md5sum)
 }
 
+// MD5Base64String returns base64 encoded MD5sum value.
+func (r *Reader) MD5Base64String() string {
+	return base64.StdEncoding.EncodeToString(r.md5sum)
+}
+
 // SHA256HexString returns hex sha256 value.
 func (r *Reader) SHA256HexString() string {
 	return hex.EncodeToString(r.sha256sum)

pkg/hash/reader_test.go

@@ -40,6 +40,9 @@ func TestHashReaderHelperMethods(t *testing.T) {
 	if r.SHA256HexString() != "88d4266fd4e6338d13b845fcf289579d209c897823b9217da3e161936f031589" {
 		t.Errorf("Expected sha256hex \"88d4266fd4e6338d13b845fcf289579d209c897823b9217da3e161936f031589\", got %s", r.SHA256HexString())
 	}
+	if r.MD5Base64String() != "4vxxTEcn7pOV8yTNLn8zHw==" {
+		t.Errorf("Expected md5base64 \"4vxxTEcn7pOV8yTNLn8zHw==\", got \"%s\"", r.MD5Base64String())
+	}
 	if r.Size() != 4 {
 		t.Errorf("Expected size 4, got %d", r.Size())
 	}