fix headers bench: use Vec::reserve for reserve buf capacity

[kanarus]

Fix response::Headers::_write_to, for-benchmark wrapper of write_unchecked_to, to reserve buf capacity by Vec::reserve instead of Vec::reserve_exact. Benchmark examples:

before

test write_02_http_crate       ... bench:         118.08 ns/iter (+/- 21.05)
test write_03_fxmap            ... bench:          94.93 ns/iter (+/- 5.96)
test write_03_header_hashbrown ... bench:          94.41 ns/iter (+/- 7.58)
test write_03_ohkami           ... bench:         153.62 ns/iter (+/- 12.09)
test write_04_headermap        ... bench:         105.58 ns/iter (+/- 18.24)

after

test write_02_http_crate       ... bench:         116.04 ns/iter (+/- 2.89)
test write_03_fxmap            ... bench:         100.20 ns/iter (+/- 4.12)
test write_03_header_hashbrown ... bench:          99.91 ns/iter (+/- 2.76)
test write_03_ohkami           ... bench:         117.10 ns/iter (+/- 3.13)
test write_04_headermap        ... bench:         124.93 ns/iter (+/- 3.37)

---

and

• Activate "inline-more" feature of hashbrown
• Introduce size tracking in header_hashbrown
• Fix input to the same & larger

for more fairness