Fix to download error in some stream addons

Without this fix, on addon gomovies, for eg, when I try to download subtitles to "lone survivor" or avatar I get this error:
00:15:52 T:1560   DEBUG: ### [Legendas.TV] - Version: 2.2.1
00:15:52 T:1560   DEBUG: ### [Legendas.TV] - Action 'download' called
00:15:52 T:1560   ERROR: EXCEPTION Thrown (PythonToCppException) : -->Python callback/script returned the following error<--
                                             - NOTE: IGNORING THIS CAN LEAD TO MEMORY LEAKS!
                                            Error Type: <type 'exceptions.KeyError'>
                                            Error Contents: ('filename',)
                                            Traceback (most recent call last):
                                              File "C:\Users\joana_000\AppData\Roaming\XBMC\addons\service.subtitles.legendastv\service.py", line 245, in <module>
                                                subs = Download(params["download_url"],params["filename"])
                                            KeyError: ('filename',)
                                            -->End of Python script error report<--

With this litle fix it downloads, I don't know if this is the best, but works.