Check for uri in segment key before creating class

[GreenZapdos]

On some m3u8 servers, there is an #EXT-X-KEY line without a uri to signify that there is not a key.

[flavioribeiro]

@GreenZapdos can you add tests and an example playlist so we can understand better your pull request?

[GreenZapdos]

The current version of m3u8 (0.2.4) will not load a playlist like this:

#EXTM3U

#EXT-X-KEY:METHOD=AES-128,URI="http://example.com/key1",IV=0x01
#EXTINF:10,
http://example.com/video/encrypted-1.ts

#EXT-X-KEY:METHOD=AES-128,URI="http://example.com/key2",IV=0x02
#EXTINF:10,
http://example.com/video/encrypted-2.ts

#EXT-X-KEY:METHOD=AES-128,URI="http://example.com/key3",IV=0x03
#EXTINF:10,
http://example.com/video/encrypted-3.ts

#EXT-X-KEY:METHOD=UNENCRYPTED
#EXTINF:10,
http://example.com/video/unencrypted-A.ts

I'm not sure how this should be tested.

[leandromoreira]

@GreenZapdos I think you can inspire by looking at https://github.com/globocom/m3u8/blob/master/tests/test_parser.py#L10 but instead of a SIMPLE_PLAYLIST you could create another type like PLAYLIST_WITH_EXTKEY_NO_URI https://github.com/globocom/m3u8/blob/master/tests/playlists.py#L10 and make sure it will pass only if your PR is applied.

[leandromoreira]

@GreenZapdos ping?