Added Proxy-Authorization to default masked header names.

src/main/java/org/gitlab4j/api/utils/MaskingLoggingFilter.java

@@ -47,7 +47,7 @@ public class MaskingLoggingFilter implements ClientRequestFilter, ClientResponse
      * Default list of header names that should be masked.
      */
     public static final List<String> DEFAULT_MASKED_HEADER_NAMES = 
-            Collections.unmodifiableList(Arrays.asList("PRIVATE-TOKEN", "Authorization"));
+            Collections.unmodifiableList(Arrays.asList("PRIVATE-TOKEN", "Authorization", "Proxy-Authorization"));
 
     /**
      * Prefix for request log entries.