Add regression test for PR-266.

tests/tester.cc

diff --git a/tests/tester.cc b/tests/tester.cc
index e7a7006..012b54d 100644
--- a/tests/tester.cc
+++ b/tests/tester.cc
@@ -412,3 +412,19 @@
   REQUIRE(true == ret);
 }
 
+#ifndef TINYGLTF_NO_FS
+TEST_CASE("expandpath-utf-8", "[pr-226]") {
+
+  std::string s1 = "\xe5\xaf\xb9"; // utf-8 string
+
+  std::string ret = tinygltf::ExpandFilePath(s1, /* userdata */nullptr);
+
+  // expected: E5 AF B9
+  REQUIRE(3 == ret.size());
+
+  REQUIRE(0xe5 == static_cast<uint8_t>(ret[0]));
+  REQUIRE(0xaf == static_cast<uint8_t>(ret[1]));
+  REQUIRE(0xb9 == static_cast<uint8_t>(ret[2]));
+
+}
+#endif