Make string construction explicit as implicit conversion doesn't work in 4.4

test/cpp/end2end/async_end2end_test.cc

@@ -532,15 +532,19 @@ TEST_F(AsyncEnd2endTest, MetadataRpc) {
   send_request.set_message("Hello");
   std::pair<grpc::string, grpc::string> meta1("key1", "val1");
   std::pair<grpc::string, grpc::string> meta2(
-      "key2-bin", {"\xc0\xc1\xc2\xc3\xc4\xc5\xc6\xc7\xc8\xc9\xca\xcb\xcc", 13});
+      "key2-bin",
+      grpc::string("\xc0\xc1\xc2\xc3\xc4\xc5\xc6\xc7\xc8\xc9\xca\xcb\xcc",
+		   13));
   std::pair<grpc::string, grpc::string> meta3("key3", "val3");
   std::pair<grpc::string, grpc::string> meta6(
       "key4-bin",
-      {"\x10\x11\x12\x13\x14\x15\x16\x17\x18\x19\x1a\x1b\x1c\x1d", 14});
+      grpc::string("\x10\x11\x12\x13\x14\x15\x16\x17\x18\x19\x1a\x1b\x1c\x1d",
+		   14));
   std::pair<grpc::string, grpc::string> meta5("key5", "val5");
   std::pair<grpc::string, grpc::string> meta4(
       "key6-bin",
-      {"\xe0\xe1\xe2\xe3\xe4\xe5\xe6\xe7\xe8\xe9\xea\xeb\xec\xed\xee", 15});
+      grpc::string("\xe0\xe1\xe2\xe3\xe4\xe5\xe6\xe7\xe8\xe9\xea\xeb\xec\xed\xee",
+		   15));
 
   cli_ctx.AddMetadata(meta1.first, meta1.second);
   cli_ctx.AddMetadata(meta2.first, meta2.second);