rpc: multicall requires methodName as the struct's first member

Faults clearly instead of the confusing positional parse error it replaces.

test/rpc/test_xmlrpc.h

@@ -11,6 +11,7 @@ class TestXmlrpc : public test_fixture {
   CPPUNIT_TEST(test_invalid_utf8);
   CPPUNIT_TEST(test_size_limit);
   CPPUNIT_TEST(test_response_size_limit);
+  CPPUNIT_TEST(test_multicall_member_order);
 
   CPPUNIT_TEST_SUITE_END();
 
@@ -24,6 +25,7 @@ public:
   void test_invalid_utf8();
   void test_size_limit();
   void test_response_size_limit();
+  void test_multicall_member_order();
 
 private:
   std::unique_ptr<TestMainThread> m_test_main_thread;