Cleanup and fixes (#3273)

* newRequestID() (previously generateUploadID()) returns string than byte array.
* Remove unclear comments and added appropriate comments.
* SHA-256, MD5 Hash functions return Hex/Base64 encoded string than byte array.
* Remove duplicate MD5 hasher functions.
* Rename listObjectsValidateArgs() into validateListObjectsArgs()
* Remove repeated auth check code in all bucket request handlers.
* Remove abbreviated names in bucket-metadata
* Avoid nested if in bucketPolicyMatchStatement()
* Use ioutil.ReadFile() instead of os.Open() and ioutil.ReadAll()
* Set crossDomainXML as constant.

cmd/auth-handler_test.go

@@ -198,7 +198,7 @@ func TestIsRequestUnsignedPayload(t *testing.T) {
 		// Test case - 2.
 		// Test case with "X-Amz-Content-Sha256" header set to  "UNSIGNED-PAYLOAD"
 		// The payload is flagged as unsigned When "X-Amz-Content-Sha256" header is set to  "UNSIGNED-PAYLOAD".
-		{"UNSIGNED-PAYLOAD", true},
+		{unsignedPayload, true},
 		// Test case - 3.
 		// set to a random value.
 		{"abcd", false},