avoid duplicate Content-Length header in DefaultClient

[alhudz]

Repro: send a body-less POST/PUT whose template already carries a Content-Length header (e.g. @Headers("Content-Length: 0")), with sun.net.http.allowRestrictedHeaders=true so the header reaches the wire.
Cause: DefaultClient.convertAndSend writes the request Content-Length in the header loop and then unconditionally adds a second Content-Length: 0 for every body-less method that allows a body, so the field goes out twice. RFC 7230 §3.3.2 forbids generating multiple Content-Length fields; the duplicate is ambiguous framing and servers reject it (Tomcat returns 400, #2862).
Fix: only add the fallback Content-Length: 0 when the request does not already declare a Content-Length.

The regression test is gated on sun.net.http.allowRestrictedHeaders like the existing Content-Length tests, since HttpURLConnection otherwise drops the restricted header, and asserts the field is sent once.