As part of the CMake port we updated the duration formatting to contain
maximum of 6 decimal places but without trailing 0s. There was a bug
however where it used 6 significant digits rather than 6 decimal places
(`%g` rather than `%f`).
This fixes the bug and also updates the MPD sample files for the
integration tests to contain maximum of 6 decimal places.
Although FRAME-RATE is optional per HLS specification, but per HLS
Authoring Specification for Apple Devices (https://apple.co/30n90DC):
Each EXT-X-STREAM-INF tag MUST have a FRAME-RATE attribute.
Also, iOS and TVos refuse to play the content if VIDEO-RANGE is
present but FRAME-RATE is missing.
Issue #632, #634.
Change-Id: Ica282f023a7e8538f7b506094e2286840cf5c193