Give network.xmlrpc.size_limit.set a floor

A limit below any valid request rejects the request that would raise it again.

test/rpc/test_rpc_manager.h

@@ -0,0 +1,17 @@
+#include "test/helpers/test_fixture.h"
+
+#include "rpc/rpc_manager.h"
+
+class TestRpcManager : public test_fixture {
+  CPPUNIT_TEST_SUITE(TestRpcManager);
+
+  CPPUNIT_TEST(test_size_limit_bounds);
+
+  CPPUNIT_TEST_SUITE_END();
+
+public:
+  void test_size_limit_bounds();
+
+private:
+  rpc::RpcManager m_rpc_manager;
+};