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/api-errors.go

@@ -572,6 +572,7 @@ func toAPIErrorCode(err error) (apiErr APIErrorCode) {
 	if err == nil {
 		return ErrNone
 	}
+
 	err = errorCause(err)
 	// Verify if the underlying error is signature mismatch.
 	switch err {
@@ -580,10 +581,12 @@ func toAPIErrorCode(err error) (apiErr APIErrorCode) {
 	case errContentSHA256Mismatch:
 		apiErr = ErrContentSHA256Mismatch
 	}
+
 	if apiErr != ErrNone {
 		// If there was a match in the above switch case.
 		return apiErr
 	}
+
 	switch err.(type) {
 	case StorageFull:
 		apiErr = ErrStorageFull
@@ -634,6 +637,7 @@ func toAPIErrorCode(err error) (apiErr APIErrorCode) {
 	default:
 		apiErr = ErrInternalError
 	}
+
 	return apiErr
 }