Optimize content type parsing

This commit avoid calling HttpHeaders#getContentType multiple times in
ServletServerHttpResponse#writeHeaders.

Closes spring-projectsgh-32339

Author: sdeleuze

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

@@ -120,12 +120,13 @@ private void writeHeaders() {
 				}
 			});
 			// HttpServletResponse exposes some headers as properties: we should include those if not already present
-			if (this.servletResponse.getContentType() == null && this.headers.getContentType() != null) {
-				this.servletResponse.setContentType(this.headers.getContentType().toString());
+			MediaType contentTypeHeader = this.headers.getContentType();
+			if (this.servletResponse.getContentType() == null && contentTypeHeader != null) {
+				this.servletResponse.setContentType(contentTypeHeader.toString());
 			}
-			if (this.servletResponse.getCharacterEncoding() == null && this.headers.getContentType() != null &&
-					this.headers.getContentType().getCharset() != null) {
-				this.servletResponse.setCharacterEncoding(this.headers.getContentType().getCharset().name());
+			if (this.servletResponse.getCharacterEncoding() == null && contentTypeHeader != null &&
+					contentTypeHeader.getCharset() != null) {
+				this.servletResponse.setCharacterEncoding(contentTypeHeader.getCharset().name());
 			}
 			long contentLength = getHeaders().getContentLength();
 			if (contentLength != -1) {