Don't allow one invalid char after header name when allowing spaces

[nox]

No description provided.

[acfoltzer]

Looks good, thank you!

Even though the sequence of operations doesn't change much with this patch, the revised code structure must really agree with some compiler optimizations:

❯ cargo criterion
   Compiling httparse v1.7.0 (/home/acfoltzer/src/httparse)
    Finished bench [optimized] target(s) in 22.59s
Gnuplot not found, using plotters backend
req/req                 time:   [249.34 ns 250.45 ns 251.62 ns]
                        thrpt:  [2.6649 GiB/s 2.6774 GiB/s 2.6893 GiB/s]
                 change:
                        time:   [-22.215% -21.847% -21.523%] (p = 0.00 < 0.05)
                        thrpt:  [+27.426% +27.954% +28.559%]
                        Performance has improved.

req_short/req_short     time:   [60.949 ns 61.140 ns 61.380 ns]
                        thrpt:  [1.0318 GiB/s 1.0358 GiB/s 1.0391 GiB/s]
                 change:
                        time:   [-4.9845% -4.5957% -4.2197%] (p = 0.00 < 0.05)
                        thrpt:  [+4.4056% +4.8171% +5.2460%]
                        Performance has improved.

resp/resp               time:   [262.45 ns 263.11 ns 263.86 ns]
                        thrpt:  [2.4672 GiB/s 2.4742 GiB/s 2.4805 GiB/s]
                 change:
                        time:   [-26.910% -26.560% -26.208%] (p = 0.00 < 0.05)
                        thrpt:  [+35.516% +36.166% +36.818%]
                        Performance has improved.

resp_short/resp_short   time:   [57.907 ns 58.072 ns 58.247 ns]
                        thrpt:  [1.4550 GiB/s 1.4594 GiB/s 1.4636 GiB/s]
                 change:
                        time:   [-20.539% -19.870% -19.219%] (p = 0.00 < 0.05)
                        thrpt:  [+23.792% +24.797% +25.848%]
                        Performance has improved.

A nice bonus in addition to the correctness fix.

[acfoltzer]

@seanmonstar I don't have write access to the repo so this will still need your +1, but lgtm.