preserve original query encoding in OrderedParams round-trip (#379)

[Mzack9999]

Fixes #379.

urlutil.OrderedParams now stores each parameter as a standalone entry that remembers the original encoded segment. Decode followed by Encode (with no mutations) produces a byte-for-byte identical query string, so parameters that came in without = stay without =, original escape choices (+ vs %20, %3D vs =) are preserved, and flipping IncludeEquals no longer mangles decoded entries - the flag only governs programmatically added/mutated parameters.

Entries touched via Add/Set/Update go through ParamEncode + IncludeEquals exactly as before; untouched siblings stay verbatim. Set and Update now replace in place at the first occurrence of the key (preserving insertion order) instead of moving the key to the end.

Validated against nuclei (pkg/fuzz/..., pkg/protocols/..., pkg/authprovider/...) and the full httpx test suite with a local replace - both build and pass.