deserializeSafeFromHeader uses context.getHeader(headerName) inst…

…ead of `context.getHeaders()` (opensearch-project#2768) (opensearch-project#2771)

(cherry picked from commit 7c4e06d)

Co-authored-by: Paras Jain <parasjain.jain@outlook.com>

src/main/java/org/opensearch/security/support/HeaderHelper.java

@@ -27,7 +27,6 @@
 package org.opensearch.security.support;
 
 import java.io.Serializable;
-import java.util.Map;
 
 import com.google.common.base.Strings;
 
@@ -56,15 +55,8 @@ public static String getSafeFromHeader(final ThreadContext context, final String
             return null;
         }
 
-        String headerValue = null;
-
-        Map<String, String> headers = context.getHeaders();
-        if (!headers.containsKey(headerName) || (headerValue = headers.get(headerName)) == null) {
-            return null;
-        }
-
         if (isInterClusterRequest(context) || isTrustedClusterRequest(context) || isDirectRequest(context)) {
-            return headerValue;
+            return context.getHeader(headerName);
         }
 
         return null;