fix: Add reserve() to Extend impl for (Option<HeaderName>, T))

[AriajSarkar]

Summary

Fixes #517 by adding reserve() to Extend<(Option<HeaderName>, T)> to match Extend<(HeaderName, T)>.

Changes

• Added reserve logic following hashbrown HashMap pattern
• Reserves full hint for empty maps, half for non-empty (handles duplicates)
• Ensures max 2 reallocations

Testing

✅ All 396 tests pass (no behavioral changes)

Code

let reserve = if self.is_empty() {
    iter.size_hint().0
} else {
    (iter.size_hint().0 + 1) / 2
};
self.reserve(reserve);