Align header checks

Author: Artur-

aws-serverless-java-container-core/src/main/java/com/amazonaws/serverless/proxy/internal/servlet/AwsHttpServletResponse.java

@@ -213,19 +213,24 @@ public void addDateHeader(String s, long l) {
     @Override
     public void setHeader(String s, String s1) {
         if (!canSetHeader()) return;
-        if ("Content-Type".equalsIgnoreCase(s)) {
+        if (isContentTypeHeader(s)) {
             setContentType(s1);
         } else {
             setHeader(s, s1, true);
         }
     }
 
 
+    private boolean isContentTypeHeader(String s) {
+        return s.toLowerCase(Locale.getDefault()).equals(HttpHeaders.CONTENT_TYPE.toLowerCase(Locale.getDefault()));
+    }
+
+
     @Override
     public void addHeader(String s, String s1) {
         if (!canSetHeader()) return;
         // TODO: We should probably have a list of headers that we are not allowed to have multiple values for
-        if (s.toLowerCase(Locale.getDefault()).equals(HttpHeaders.CONTENT_TYPE.toLowerCase(Locale.getDefault()))) {
+        if (isContentTypeHeader(s)) {
             setContentType(s1);
         } else {
             setHeader(s, s1, false);