fix: preserve Content-Type when sending JSON data with a single custom header

[lingxiu58]

Problem

When sending a JSON request with exactly one custom header, the Content-Type: application/json header is silently dropped. With two or more custom headers, Content-Type is correctly preserved.

Reproduce:

# Broken — no Content-Type header
https post pie.dev/post -v 'header1: xyz' x=1

# Works — Content-Type: application/json present
https post pie.dev/post -v 'header1: xyz' header2: abc x=1

In both cases the data IS serialized as JSON, but the server doesn't know to parse it as JSON in the single-header case.

Root Cause

apply_missing_repeated_headers() in client.py uses CIMultiDict.popone() + update() to re-add original headers after requests.PreparedRequest processing. When initialized from a CaseInsensitiveDict (from requests), this pop-and-replace pattern on CIMultiDict can silently lose entries depending on the internal hash table state — which correlates with the total number of header entries.

Fix

Rebuild the headers dict from scratch using add() instead of the fragile popone() + update() pattern:

1. Materialize items to lists before iteration — avoids lazy iterator issues
2. Build new_headers from empty — instead of HTTPHeadersDict(prepared_request.headers) then popping/re-adding
3. Use add() instead of popone()+update() — avoids the fragile pop-then-replace pattern on CIMultiDict

Closes #1834