Support multiple #EXT-X-KEY tags

[bwjun-tving]

Why need this change?

• When parsing and then building the m3u8 below, some #EXT-X-KEY tags are missing.

#EXTM3U
#EXT-X-VERSION:6
#EXT-X-TARGETDURATION:11
#EXT-X-PLAYLIST-TYPE:VOD
#EXT-X-MAP:URI="5d9aec6dea24.mp4",BYTERANGE="3327@0"
#EXTINF:4.338,
#EXT-X-BYTERANGE:61342@3383
5d9aec6dea24.mp4
#EXTINF:4.505,
#EXT-X-BYTERANGE:83830
5d9aec6dea24.mp4
#EXT-X-DISCONTINUITY
#EXT-X-MAP:URI="8b94105ce572.mp4",BYTERANGE="4102@0"
#EXT-X-KEY:METHOD=SAMPLE-AES,URI="data:text/plain;charset=UTF-8;base64,U...A==",IV=0x123456789abcdef123456789abcdef,KEYFORMATVERSIONS="1",KEYFORMAT="com.microsoft.playready"
#EXT-X-KEY:METHOD=SAMPLE-AES,URI="data:text/plain;base64,A...Y=",KEYID=0x123456789abcdef123456789abcdef,IV=0x123456789abcdef123456789abcdef,KEYFORMATVERSIONS="1",KEYFORMAT="urn:uuid:12345678-9abc-def0-1234-123456789abc"
#EXT-X-KEY:METHOD=SAMPLE-AES,URI="skd://X...Q==",KEYFORMATVERSIONS="1",KEYFORMAT="com.apple.streamingkeydelivery"
#EXTINF:6.506,
#EXT-X-BYTERANGE:40676@12570
8b94105ce572.mp4
#EXTINF:4.463,
#EXT-X-BYTERANGE:105347
8b94105ce572.mp4
...

Changes made:

• Support multiple #EXT-X-KEY tags