Fix #6493 header cache memory usage

Delay creating a header cache until a second request on a parser.

Signed-off-by: Greg Wilkins <gregw@webtide.com>

jetty-http/src/main/java/org/eclipse/jetty/http/HttpParser.java

@@ -1935,6 +1935,7 @@ protected void setState(FieldState state)
 
     public Index<HttpField> getFieldCache()
     {
+        checkHeaderCache();
         return _fieldCache;
     }