Set content length on ServletHttpResponse

Prior to this commit, the `ServletServerHttpResponse` would copy headers
from the `HttpHeaders` map and calls methods related to headers exposed
as properties (content-type, character encoding).

Unlike its reactive variant, this would not set the content length.
Depending on the Servlet container implementation, this could cause
duplicate Content-Length response headers in the actual HTTP response.

This commit aligns both implementations and ensures that the
`setContentLengthLong` method is called if necessary so that the Servlet
container can ensure a single header for that.

Fixes gh-26330

Author: bclozel

spring-web/src/main/java/org/springframework/http/server/ServletServerHttpResponse.java

@@ -125,6 +125,10 @@ private void writeHeaders() {
 					this.headers.getContentType().getCharset() != null) {
 				this.servletResponse.setCharacterEncoding(this.headers.getContentType().getCharset().name());
 			}
+			long contentLength = getHeaders().getContentLength();
+			if (contentLength != -1) {
+				this.servletResponse.setContentLengthLong(contentLength);
+			}
 			this.headersWritten = true;
 		}
 	}