Answer an oversized RPC response with a fault

Cap XML-RPC and JSON-RPC output at the SCGI response limit before handing it over.

test/rpc/test_xmlrpc.h

@@ -10,6 +10,7 @@ class TestXmlrpc : public test_fixture {
   CPPUNIT_TEST(test_basics);
   CPPUNIT_TEST(test_invalid_utf8);
   CPPUNIT_TEST(test_size_limit);
+  CPPUNIT_TEST(test_response_size_limit);
 
   CPPUNIT_TEST_SUITE_END();
 
@@ -22,6 +23,7 @@ public:
   void test_basics();
   void test_invalid_utf8();
   void test_size_limit();
+  void test_response_size_limit();
 
 private:
   std::unique_ptr<TestMainThread> m_test_main_thread;