Make URL normalization idempotent

[binggao1230]

NormalizeURLString/NormalizeURL are not idempotent for the library's own recommended greedy flag sets: Normalize(Normalize(x)) != Normalize(x). Since purell exists to produce stable dedup/cache keys, idempotence is a defining property, and the value a second pass reaches is exactly the one already intended (all trailing slashes stripped) — so this is a convergence bug, not a behaviour change.

Two causes, both in purell.go:

1. removeTrailingSlash stripped a single trailing slash, so a path ending in two or more slashes needed several passes to collapse: http://host// → http://host/ → http://host. This breaks idempotence with the standalone FlagRemoveTrailingSlash.
2. NormalizeURL applied the flag pass once. Flags such as removeDirectoryIndex run before removeTrailingSlash, and %2F decodes to / on parse, so a trailing slash can hide a segment a later pass would remove: http://host/index.html/ (UnsafeGreedy) → http://host/index.html → http://host.

Minimal repro:

n1, _ := purell.NormalizeURLString("http://host//", purell.FlagRemoveTrailingSlash) // "http://host/"
n2, _ := purell.NormalizeURLString(n1, purell.FlagRemoveTrailingSlash)              // "http://host"  (!= n1)

Fix: removeTrailingSlash strips every trailing slash (a root / still collapses to ""), and NormalizeURL iterates the flag pass to a bounded fixed point.

The converged output equals the value repeated normalization already reached, so no existing expected value changes — the full existing suite passes unmodified. A property fuzz over ~325k random URLs per flag set goes from:

FlagsUsuallySafeGreedy   not-idempotent 13798 -> 0
FlagsUnsafeGreedy        not-idempotent   232 -> 0
FlagsSafe, *NonGreedy    already 0

TestNormalizeIsIdempotent covers the standalone flag, both greedy sets, and the directory-index / %2F sub-cases.