Fix to_string(time_t) and write a unit test for it

tests/utils.cpp

@@ -7,6 +7,7 @@
 #include <utils/xdg.hpp>
 #include <utils/empty_if_fixed_server.hpp>
 #include <utils/get_first_non_empty.hpp>
+#include <utils/time.hpp>
 
 using namespace std::string_literals;
 
@@ -111,3 +112,12 @@ TEST_CASE("first non-empty string")
   CHECK(get_first_non_empty("first"s) == "first"s);
   CHECK(get_first_non_empty(0, 1, 2, 3) == 1);
 }
+
+TEST_CASE("time_to_string")
+{
+  const std::time_t stamp = 1472480968;
+  const std::string result = "2016-08-29T14:29:28Z";
+  CHECK(utils::to_string(stamp) == result);
+  CHECK(utils::to_string(stamp).size() == result.size());
+  CHECK(utils::to_string(0) == "1970-01-01T00:00:00Z");
+}