fixed issue #173 - timezone as output by %z option has a double negative sign

include/spdlog/details/pattern_formatter_impl.h

@@ -317,8 +317,10 @@ public:
 
         int h = total_minutes / 60;
         int m = total_minutes % 60;
-        char sign = h >= 0 ? '+' : '-';
-        msg.formatted << sign;
+        if (h >= 0) //minus sign will be printed anyway if negative
+        {
+            msg.formatted << '+';
+        }
         pad_n_join(msg.formatted, h, m, ':');
     }
 private: