Calculate correct S3 Transfer key with or without prefix (fixes aws#653)

Resolve an issue where S3 Transfer was attempting to upload objects with
an empty key when uploading to the root of an S3 bucket.

Also ensures that there is no `//` between prefix and path, to avoid uploading
objects one level deeper than expected inside a directory with empty name.

src/S3/Transfer.php

@@ -198,7 +198,7 @@ private function determineScheme($path)
     }
 
     /**
-     * Normalize a path so that it has a trailing slash.
+     * Normalize a path so that it has UNIX-style directory separators and no trailing /
      *
      * @param string $path
      *
@@ -317,15 +317,16 @@ private function uploadMultipart($filename)
 
     private function createS3Key($filename)
     {
-        if (!isset($this->s3Args['Key'])) {
-            return '';
+        $relative_file_path = ltrim(
+            preg_replace('#^' . preg_quote($this->source['path']) . '#', '', $filename),
+            '/'
+        );
+
+        if (isset($this->s3Args['Key'])) {
+            return rtrim($this->s3Args['Key']), '/').'/'.$relative_file_path;
+        } else {
+            return $relative_file_path;
         }
-
-        $args = $this->s3Args;
-        $args['Key'] = rtrim($args['Key'], '/');
-        $args['Key'] .= preg_replace('#^' . preg_quote($this->source['path']) . '#', '', $filename);
-
-        return $args['Key'];
     }
 
     private function addDebugToBefore($debug)