XL: Handle quorum situations properly for write operations. (#1986)

Adds two test cases one for

 - PutObject when write quorum is not available.
 - PutObjectPart when write quorum is not available.

Fixes #1951

benchmark-utils_test.go

@@ -86,7 +86,7 @@ func generateBytesData(size int) []byte {
 		b = letterBytes[rand.Intn(len(letterBytes))]
 		return []byte{b}
 	}
-	// repeat the random character chosen size.
+	// repeat the random character chosen size
 	return bytes.Repeat(getRandomByte(), size)
 }