Remove Content-Type Header when request with empty body POST method (Default Client).

- add Content-Length Header with 0 value when `sun.net.http.allowRestrictedHeaders` System Property is set true

- fix not running test to run

- add README for setting zero content-length header

Author: jay-choe

README.md

@@ -815,6 +815,12 @@ in a context-specific manner -- for example, thread-local storage can be used to
 header values depending on the invoking thread, which can be useful for things such as setting
 thread-specific trace identifiers for requests.
 
+#### Set zero Content-Length Header
+
+To specify `Content-Length: 0` header when making a request with empty body, system property `sun.net.http.allowRestrictedHeaders` should be set to `true`
+
+If not, the `Content-Length` header will not be added.
+
 ### Advanced usage
 
 #### Base Apis

core/src/main/java/feign/Client.java

@@ -201,19 +201,14 @@ else if (field.equals(ACCEPT_ENCODING)) {
         connection.addRequestProperty("Accept", "*/*");
       }
 
-      boolean hasEmptyBody = false;
       byte[] body = request.body();
-      if (body == null && request.httpMethod().isWithBody()) {
-        body = new byte[0];
-        hasEmptyBody = true;
-      }
 
       if (body != null) {
         /*
          * Ignore disableRequestBuffering flag if the empty body was set, to ensure that internal
          * retry logic applies to such requests.
          */
-        if (disableRequestBuffering && !hasEmptyBody) {
+        if (disableRequestBuffering) {
           if (contentLength != null) {
             connection.setFixedLengthStreamingMode(contentLength);
           } else {
@@ -236,6 +231,12 @@ else if (field.equals(ACCEPT_ENCODING)) {
           }
         }
       }
+
+      if (body == null && request.httpMethod().isWithBody()) {
+        // To use this Header, set 'sun.net.http.allowRestrictedHeaders' property true.
+        connection.addRequestProperty("Content-Length", "0");
+      }
+
       return connection;
     }
 

core/src/test/java/feign/client/DefaultClientTest.java

@@ -28,6 +28,7 @@
 import feign.assertj.MockWebServerAssertions;
 import okhttp3.mockwebserver.MockResponse;
 import okhttp3.mockwebserver.SocketPolicy;
+import org.junit.jupiter.api.condition.EnabledIfSystemProperty;
 
 /**
  * Tests client-specific behavior, such as ensuring Content-Length is sent when specified.
@@ -79,17 +80,37 @@ public void patch() throws Exception {
     assertThat(exception).hasCauseInstanceOf(ProtocolException.class);
   }
 
+  @Test
   @Override
   public void noResponseBodyForPost() throws Exception {
     super.noResponseBodyForPost();
     MockWebServerAssertions.assertThat(server.takeRequest()).hasMethod("POST")
+        .hasNoHeaderNamed("Content-Type");
+  }
+
+  @Test
+  @EnabledIfSystemProperty(named = "sun.net.http.allowRestrictedHeaders", matches = "true")
+  public void noRequestBodyForPostWithAllowRestrictedHeaders() throws Exception {
+    super.noResponseBodyForPost();
+    MockWebServerAssertions.assertThat(server.takeRequest()).hasMethod("POST")
+        .hasNoHeaderNamed("Content-Type")
         .hasHeaders(entry("Content-Length", Collections.singletonList("0")));
   }
 
+  @Test
   @Override
   public void noResponseBodyForPut() throws Exception {
     super.noResponseBodyForPut();
     MockWebServerAssertions.assertThat(server.takeRequest()).hasMethod("PUT")
+        .hasNoHeaderNamed("Content-Type");
+  }
+
+  @Test
+  @EnabledIfSystemProperty(named = "sun.net.http.allowRestrictedHeaders", matches = "true")
+  public void noResponseBodyForPutWithAllowRestrictedHeaders() throws Exception {
+    super.noResponseBodyForPut();
+    MockWebServerAssertions.assertThat(server.takeRequest()).hasMethod("PUT")
+        .hasNoHeaderNamed("Content-Type")
         .hasHeaders(entry("Content-Length", Collections.singletonList("0")));
   }