Update Gap detection in NextFragmentRequestRule

Created by: nicosang

Hi,

this PR has to solve issue detected by @dsparacio in dashjs slack channel.

In 2.7.0 I added enableForcedTextStreaming and it is working as expected in 2.7.0. However in 2.9.0 it still works upfront, but on a seek the text fragment is loaded over and over again rapidly and fills the text track with duplication of captions

The bug could also occur by doing a simple seek command.

NextFragmentRequestRule has not to detect gap in buffers for fragmented Text media type because those buffers are never deleted when, for instance, a seek command occurs.

Nico