Update Content-Length if body changed by client interceptor

Prior to this commit, the HTTP interceptor model used for `RestTemplate`
and `RestClient` would not update the "Content-Length" request header,
even when the request body had been updated by a
`ClientHttpRequestInterceptor`.

Even though this is the `ClientHttpRequestInterceptor`'s responsibility
(along with the content type and encoding changes if needed), this
would result in invalid requests. This invalid situation can be detected
by `InterceptingClientHttpRequest`.

This commit ensures that such situations are detected and fixed
automatically by setting the Content-Length header to the actual body
size, right before executing the actual request, after all interceptors
are done.

Closes gh-33459

spring-web/src/main/java/org/springframework/http/client/InterceptingClientHttpRequest.java

@@ -93,6 +93,10 @@ public ClientHttpResponse execute(HttpRequest request, byte[] body) throws IOExc
 				request.getHeaders().forEach((key, value) -> delegate.getHeaders().addAll(key, value));
 				request.getAttributes().forEach((key, value) -> delegate.getAttributes().put(key, value));
 				if (body.length > 0) {
+					long contentLength = delegate.getHeaders().getContentLength();
+					if (contentLength > -1 && contentLength != body.length) {
+						delegate.getHeaders().setContentLength(body.length);
+					}
 					if (delegate instanceof StreamingHttpOutputMessage streamingOutputMessage) {
 						streamingOutputMessage.setBody(new StreamingHttpOutputMessage.Body() {
 							@Override

spring-web/src/test/java/org/springframework/http/client/InterceptingClientHttpRequestFactoryTests.java

@@ -204,6 +204,7 @@ void changeBody() throws Exception {
 		ClientHttpRequest request = requestFactory.createRequest(URI.create("https://example.com"), HttpMethod.GET);
 		request.execute();
 		assertThat(Arrays.equals(changedBody, requestMock.getBodyAsBytes())).isTrue();
+		assertThat(requestMock.getHeaders().getContentLength()).isEqualTo(changedBody.length);
 	}