Preserve empty lines in the response (#32)

Without empty lines the responses are invalid and cannot be parsed by mime parsers.

src/Connection/Connection.php

@@ -159,7 +159,7 @@ private function getMultiLineResponse(Response $response)
 
             // Check if the line terminates the text response.
             if ($line === '.') {
-                return new MultiLineResponse($response, array_filter($lines));
+                return new MultiLineResponse($response, $lines);
             }
 
             // If 1st char is '.' it's doubled (NNTP/RFC977 2.4.1).