Improve response writer safety

[tigerwill90]

goos: linux
goarch: amd64
pkg: github.com/tigerwill90/fox
cpu: Intel(R) Core(TM) i9-9900K CPU @ 3.60GHz
                    │   old.txt    │               new.txt               │
                    │    sec/op    │    sec/op     vs base               │
StaticAll-16          11.25µ ±  0%   11.12µ ±  1%  -1.14% (p=0.001 n=10)
GithubParamsAll-16    81.85n ±  1%   81.91n ±  1%       ~ (p=0.756 n=10)
OverlappingRoute-16   95.47n ±  1%   98.48n ±  1%  +3.15% (p=0.000 n=10)
StaticParallel-16     10.31n ±  5%   10.43n ±  3%       ~ (p=0.955 n=10)
CatchAll-16           31.40n ±  1%   30.33n ±  5%       ~ (p=0.093 n=10)
CatchAllParallel-16   5.661n ± 17%   5.630n ± 27%       ~ (p=0.280 n=10)
CloneWith-16          69.57n ±  2%   64.94n ±  1%  -6.65% (p=0.000 n=10)
geomean               73.16n         72.35n        -1.10%

[codecov]

Codecov Report

Attention: Patch coverage is 52.77778% with 17 lines in your changes are missing coverage. Please review.

Project coverage is 87.89%. Comparing base (6b8d3ab) to head (7720509).

Files	Patch %	Lines
response_writer.go	50.00%	14 Missing and 3 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #30      +/-   ##
==========================================
+ Coverage   86.84%   87.89%   +1.04%     
==========================================
  Files          12       12              
  Lines        1657     1660       +3     
==========================================
+ Hits         1439     1459      +20     
+ Misses        187      170      -17     
  Partials       31       31              

Flag	Coverage Δ
coverage.txt	87.89% <52.77%> (+1.04%)	⬆️

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