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/test/java/org/springframework/http/HttpHeadersTests.java

@@ -154,6 +154,17 @@ class HttpHeadersTests {
 		assertThat(headers.getFirst("Content-Length")).as("Invalid Content-Length header").isEqualTo("42");
 	}
 
+	@Test
+	void setContentLengthWithNegativeValue() {
+		assertThatIllegalArgumentException().isThrownBy(() ->
+				headers.setContentLength(-1));
+	}
+
+	@Test
+	void getContentLengthReturnsMinusOneForAbsentHeader() {
+		assertThat(headers.getContentLength()).isEqualTo(-1);
+	}
+
 	@Test
 	void contentType() {
 		MediaType contentType = new MediaType("text", "html", StandardCharsets.UTF_8);