INT-4135: Ignore case check for HTTP Content-Type

JIRA: https://jira.spring.io/browse/INT-4135

The incoming `Content-Type` HTTP header can be in any arbitrary case.

Fix `DefaultHttpHeaderMapper` `Content-Type` header re-mapping to the `MessageHeaders.CONTENT_TYPE` via `equalsIgnoreCase()` comparison

**Cherry-pick to 4.3.x**

spring-integration-http/src/main/java/org/springframework/integration/http/support/DefaultHttpHeaderMapper.java

@@ -462,7 +462,7 @@ public class DefaultHttpHeaderMapper implements HeaderMapper<HttpHeaders>, BeanF
 						if (logger.isDebugEnabled()) {
 							logger.debug(MessageFormat.format("setting headerName=[{0}], value={1}", name, value));
 						}
-						if (CONTENT_TYPE.equals(name)) {
+						if (CONTENT_TYPE.equalsIgnoreCase(name)) {
 							name = MessageHeaders.CONTENT_TYPE;
 						}
 						this.setMessageHeader(target, name, value);