SI-8690 BufferedSource.mkString mistakenly skipped the first char.

mkString is overriden in BufferedSource for performance, but the
implementation always used the wrong reader. This seems to be a typo
(`allReader` is declared 5 lines earlier but never used, `charReader`
is used in its place).
Merge pull request #3849 from gourlaysama/wip/t8690

SI-8690 BufferedSource.mkString mistakenly skipped the first char.