headers: Strictly don't allow NUL bytes
In the past (2015) this was allowed for some problematic sites. However Chromium also does not allow NUL bytes in either header names or values these days. So this should no longer be a problem.
In the past (2015) this was allowed for some problematic sites. However Chromium also does not allow NUL bytes in either header names or values these days. So this should no longer be a problem.