refactor: Replace HexStr(o.begin(), o.end()) with HexStr(o)

HexStr can be called with anything that bas `begin()` and `end()` functions,
so clean up the redundant calls.

src/test/sighash_tests.cpp

@@ -141,7 +141,7 @@ BOOST_AUTO_TEST_CASE(sighash_test)
         ss << txTo;
 
         std::cout << "\t[\"" ;
-        std::cout << HexStr(ss.begin(), ss.end()) << "\", \"";
+        std::cout << HexStr(ss) << "\", \"";
         std::cout << HexStr(scriptCode) << "\", ";
         std::cout << nIn << ", ";
         std::cout << nHashType << ", \"";