MINOR: h1: rearrange the header value parser to better deal with CTL chars

MINOR: h1: rearrange the header value parser to better deal with CTL chars

The header value parser only made a special case of NUL, CR and LF in
its core loop, which doesn't ease detection of other CTL chars. Let's
first slightly rearrange the loop to make the special case of chars below
0x20 first. The checks were only moved inside a new "if" block but nothing
was changed. Use diff -b to better see.