Merge pull request #75 from gabm/FixMissingLocale

Problem: date::format(..) doesn't compile

date.h

@@ -39,6 +39,8 @@
 #include <ostream>
 #include <ratio>
 #include <type_traits>
+#include <sstream>
+#include <cctype>
 
 namespace date
 {
@@ -4062,7 +4064,7 @@ format(std::basic_string<CharT, Traits> fmt, sys_time<Duration> tp)
 {
     const std::string abbrev("UTC");
     CONSTDATA std::chrono::seconds offset{0};
-    return detail::format(std::move(fmt), local_time<Duration>{tp.time_since_epoch()},
+    return detail::format(std::locale{}, std::move(fmt), local_time<Duration>{tp.time_since_epoch()},
                           &abbrev, &offset);
 }