Only add a Content-Type when it's not nil

Makes it possible to create MIME parts for non-file POST parameters when
sending a file.

lib/rex/mime/message.rb

@@ -88,7 +88,9 @@ class Message
 			part.header.set("Content-Disposition", content_disposition)
 		end
 
-		part.header.set("Content-Type", content_type)
+		if (content_type)
+			part.header.set("Content-Type", content_type)
+		end
 
 		if (transfer_encoding)
 			part.header.set("Content-Transfer-Encoding", transfer_encoding)