MutableHttpFields.asImmutable avoids copy

Avoid a copy in MutableHttpFields.asImmutable if the mutable is never mutated again.

Author: gregw

jetty-core/jetty-http/src/main/java/org/eclipse/jetty/http/MutableHttpFields.java

@@ -192,6 +192,11 @@ private void copyImmutable()
     @Override
     public Mutable clear()
     {
+        if (_immutable)
+        {
+            _fields = new HttpField[_fields.length];
+            _immutable = false;
+        }
         _size = 0;
         return this;
     }