Fixed bug in getDecodedContent with 'format=flowed' email

With ```Content-Type: text/plain; charset=ISO-8859-1; format=flowed``` i got some a problem. 

There are no email ```parts```, ```partNumber``` is null, so ```Message::getBodyText()``` fail back to ```getDecodedContent()```, and the function get the whole email, including the header.

Using ```1``` as ```partNumber``` solved it.