Added signature validation test

tests/test_jwt_decode.cc

@@ -0,0 +1,23 @@
+#include <iostream>
+#include "gtest/gtest.h"
+#include "jwt/jwt.hpp"
+
+TEST (DecodeTest, InvalidFinalDotForNoneAlg)
+{
+  using namespace jwt::params;
+  const char* inv_enc_str = 
+    "eyJhbGciOiJOT05FIiwidHlwIjoiSldUIn0.eyJhdWQiOiJyaWZ0LmlvIiwiZXhwIjoxNTEzODYzMzcxLCJzdWIiOiJub3RoaW5nIG11Y2gifQ";
+
+  std::error_code ec;
+  auto obj = jwt::decode(inv_enc_str, "", algorithms({"none", "hs256"}), ec);
+
+  ASSERT_TRUE (ec);
+  EXPECT_EQ (ec.value(), static_cast<int>(jwt::DecodeErrc::SignatureFormatError));
+}
+
+
+int main(int argc, char* argv[]) {
+  ::testing::InitGoogleTest(&argc, argv);
+  return RUN_ALL_TESTS();
+  return 0;
+}