Make ImapMailReceiverTests#testAttachments() OS independent

The line of the test ImapMailReceiverTests#testAttachments():

assertEquals("bar\n", ((Multipart) content).getBodyPart(0).getContent());

causes an issue on Windows, beacause the read of 'test.mail' resource and copy of it into OutputStream converts line-separators to OS specific. So, it returns 'bar\t\n'