Fixes #58 - Error assertion in NewlineBetweenPayloadText

Assert that error occurs at the beginning line 7 rather than line 3

The test should encounter a non-fatal error resulting in a dropped cue
(WEBVTT_CUE_INCOMPLETE, on line 7 column 1), due to encountering 2
sequential EOL sequences prior to WebVTT cue-times.

The test comment has been updated to make a note of this.