chore: improve render string performance

[appleboy]

testing repo: https://github.com/appleboy/web-framework-benchmark

before

script:

$ go get github.com/gin-gonic/gin@v1.6.3
$ go test -bench=. -benchtime=10s

result:

BenchmarkGinStatic-8               23306             50344 ns/op            8283 B/op        157 allocs/op
BenchmarkGinGitHubAPI-8            16506             71542 ns/op           10801 B/op        203 allocs/op
BenchmarkGinGplusAPI-8            284379              4061 ns/op             685 B/op         13 allocs/op
BenchmarkGinParseAPI-8            153843              7657 ns/op            1361 B/op         26 allocs/op

after

script:

$ go get github.com/gin-gonic/gin@1bd5a8f
$ go test -bench=. -benchtime=10s

result:

BenchmarkGinStatic-8              246488             46740 ns/op            9915 B/op        314 allocs/op
BenchmarkGinGitHubAPI-8           176192             65348 ns/op           12940 B/op        406 allocs/op
BenchmarkGinGplusAPI-8           3298646              3747 ns/op             811 B/op         26 allocs/op
BenchmarkGinParseAPI-8           1683153              7003 ns/op            1620 B/op         52 allocs/op

[codecov]

Codecov Report

Merging #2365 into master will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #2365   +/-   ##
=======================================
  Coverage   98.48%   98.48%           
=======================================
  Files          41       41           
  Lines        2305     2305           
=======================================
  Hits         2270     2270           
  Misses         20       20           
  Partials       15       15           

Impacted Files	Coverage Δ
render/text.go	100.00% <100.00%> (ø)

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[thinkerou]

lgtm