Adds option for setting custom content type for modify request body.

fixes gh-492

spring-cloud-gateway-sample/src/main/java/org/springframework/cloud/gateway/sample/GatewaySampleApplication.java

@@ -70,10 +70,8 @@ public class GatewaySampleApplication {
 				)
 				.route("rewrite_request_obj", r -> r.host("*.rewriterequestobj.org")
 					.filters(f -> f.prefixPath("/httpbin")
-							//TODO: add as configuration to modifyRequestBody
-							.setRequestHeader("Content-Type", MediaType.APPLICATION_JSON_VALUE)
 							.addResponseHeader("X-TestHeader", "rewrite_request")
-							.modifyRequestBody(String.class, Hello.class,
+							.modifyRequestBody(String.class, Hello.class, MediaType.APPLICATION_JSON_VALUE,
 									(exchange, s) -> {
 										return Mono.just(new Hello(s.toUpperCase()));
 									})