set self.is_known_utf8 to false in extend_from_slice
try-job: x86_64-msvc
closes #126977
Related to #126885, #126333, and [this conversation](<aa46a3368e (r143539097)
>)
set self.is_known_utf8 to false in extend_from_slice
try-job: x86_64-msvc
closes #126977
Related to #126885, #126333, and [this conversation](<aa46a3368e (r143539097)
>)