♻️ Refactor: improve iterator performance. (#3562)

* ♻️ Refactor: improve iterator performance.

```
goos: darwin
goarch: arm64
pkg: github.com/gofiber/fiber/v3/client
cpu: Apple M4
                        │   old.txt    │           new.txt           │
                        │    sec/op    │   sec/op     vs base        │
_Request_Headers-10       140.15n ± 0%   99.39n ± 1%  -29.08% (n=50)
_Request_Params-10        135.70n ± 0%   41.01n ± 0%  -69.78% (n=50)
_Request_AllFormData-10   135.60n ± 0%   94.67n ± 0%  -30.18% (n=50)
_Headers-10                291.3n ± 0%   156.8n ± 1%  -46.17% (n=50)
geomean                    165.6n        88.20n       -46.73%

                        │   old.txt   │              new.txt               │
                        │    B/op     │    B/op     vs base                │
_Request_Headers-10        200.0 ± 0%   184.0 ± 0%   -8.00% (n=50)
_Request_Params-10        232.00 ± 0%   88.00 ± 0%  -62.07% (n=50)
_Request_AllFormData-10    232.0 ± 0%   184.0 ± 0%  -20.69% (n=50)
_Headers-10                296.0 ± 0%   376.0 ± 0%  +27.03% (p=0.000 n=50)
geomean                    237.6        182.9       -23.00%

                        │  old.txt   │          new.txt           │
                        │ allocs/op  │ allocs/op   vs base        │
_Request_Headers-10       7.000 ± 0%   6.000 ± 0%  -14.29% (n=50)
_Request_Params-10        9.000 ± 0%   4.000 ± 0%  -55.56% (n=50)
_Request_AllFormData-10   9.000 ± 0%   6.000 ± 0%  -33.33% (n=50)
_Headers-10               9.000 ± 0%   6.000 ± 0%  -33.33% (n=50)
geomean                   8.452        5.422       -35.85%
```

* 🩹 Fix: fix wrong logic and revert Headers()