Avoid default Content-Type on POST/PUT/PATCH with empty body - #3507

Fixes #2068

Problem

DefaultClient opens the connection's output stream and writes to it whenever the request body is non-null, even if it is a zero-length array. HttpURLConnection creates an internal "poster" as soon as an output stream is requested, and defaults Content-Type to application/x-www-form-urlencoded if none was explicitly set — regardless of whether any bytes are actually written. This is undocumented JDK behavior (see the JDK's HttpURLConnection source referenced in the issue).

This causes POST/PUT/PATCH requests with an empty body (e.g. an empty String argument, which encodes to a non-null zero-length byte[]) to silently get an incorrect Content-Type header, which can make servers reject the request with 415 Unsupported Media Type.

#2555 partially addressed this by no longer synthesizing a fake empty byte[] for a null body (avoiding the output stream in that specific case), but it left the output stream path in place for any body that is non-null with length == 0 — which is exactly what happens whenever a caller explicitly sends an empty body. That's why the issue was left open after #2555 merged.

Fix

Skip connection.getOutputStream() entirely whenever the body is empty (null or zero-length) for methods that carry a body (POST/PUT/PATCH), and set Content-Length: 0 directly instead — mirroring the existing null-body path. Non-empty bodies are unaffected.

Testing

Added emptyStringBodyForPost() to AbstractClientTest/DefaultClientTest, asserting a POST with an explicit empty-string body does not get a Content-Type header. Confirmed the test fails on master (reproducing the reported header) and passes with this change.

Ran the full test suite for core, java11, googlehttpclient, okhttp, hc5, httpclient, and jaxrs2 (all modules sharing AbstractClientTest) — all green, no regressions in other Client implementations.

Verification done:

1. Checked for an in-flight PR/duplicate — none open against POST requests with empty body have "Content-Type: application/x-www-form-urlencoded" header since Feign 12.0 #2068 besides the already-merged, partial Remove Content-Type Header when request with empty body POST method #2555.
2. No active self-claim on the issue.
3. Change is code + tests only (.java), no docs.
4. Confirmed on current master (grepped DefaultClient.convertAndSend) that the empty-non-null-body path still calls getOutputStream()/write(), and reproduced the bug with a new test before applying the fix.
5. No parent epic.
6. Not a spring-projects/* repo, so the triage gate doesn't apply; issue already carries bug + help wanted labels.