Properly set mtime on S3 for touch operation

The code was missing the "MetadataDirective".
Once added, some other parts of the code failed because the format of mtime was wrong.
So this PR uses the RFC format that the S3 library already uses.

Additionally, the code path where mtime is null was missing. Now defaulting to
the current time.

apps/files_external/lib/amazons3.php

@@ -439,9 +439,12 @@ class AmazonS3 extends \OC\Files\Storage\Common {
 		$path = $this->normalizePath($path);
 
 		$metadata = array();
-		if (!is_null($mtime)) {
-			$metadata = array('lastmodified' => $mtime);
+		if (is_null($mtime)) {
+			$mtime = time();
 		}
+		$metadata = [
+			'lastmodified' => gmdate(\Aws\Common\Enum\DateFormat::RFC1123, $mtime)
+		];
 
 		$fileType = $this->filetype($path);
 		try {
@@ -449,22 +452,24 @@ class AmazonS3 extends \OC\Files\Storage\Common {
 				if ($fileType === 'dir' && ! $this->isRoot($path)) {
 					$path .= '/';
 				}
-				$this->getConnection()->copyObject(array(
+				$this->getConnection()->copyObject([
 					'Bucket' => $this->bucket,
 					'Key' => $this->cleanKey($path),
 					'Metadata' => $metadata,
-					'CopySource' => $this->bucket . '/' . $path
-				));
+					'CopySource' => $this->bucket . '/' . $path,
+					'MetadataDirective' => 'REPLACE',
+				]);
 				$this->testTimeout();
 			} else {
 				$mimeType = \OC_Helper::getMimetypeDetector()->detectPath($path);
-				$this->getConnection()->putObject(array(
+				$this->getConnection()->putObject([
 					'Bucket' => $this->bucket,
 					'Key' => $this->cleanKey($path),
 					'Metadata' => $metadata,
 					'Body' => '',
-					'ContentType' => $mimeType
-				));
+					'ContentType' => $mimeType,
+					'MetadataDirective' => 'REPLACE',
+				]);
 				$this->testTimeout();
 			}
 		} catch (S3Exception $e) {