Reject negative Content-Length values in HttpHeaders

Prior to this commit, `HttpHeaders#setContentLength` would accept
negative values. Those are not allowed by the RFC and the headers
implementation only uses "-1" as a way to convey that no value was set.

This commit ensures that negative values are rejected.

Fixes gh-32660

spring-web/src/main/java/org/springframework/http/HttpHeaders.java

@@ -969,8 +969,13 @@ public class HttpHeaders implements MultiValueMap<String, String>, Serializable
 	/**
 	 * Set the length of the body in bytes, as specified by the
 	 * {@code Content-Length} header.
+	 * @param contentLength content length (greater than or equal to zero)
+	 * @throws IllegalArgumentException if the content length is negative
 	 */
 	public void setContentLength(long contentLength) {
+		if (contentLength < 0) {
+			throw new IllegalArgumentException("Content-Length must be a non-negative number");
+		}
 		set(CONTENT_LENGTH, Long.toString(contentLength));
 	}