When a non-DST time is passed to the httpd_util:rfc1123_date/1 function, it

causes a case-clause error, as the calender:local_time_to_universal_time_dst/1
can return an empty list, which is not currently handled.

When called with an invalid DST time:
1> httpd_util:rfc1123_date({{2017, 03, 26},{1, 0, 0}}).
   ** exception error: no case clause matching []
      in function  httpd_util:rfc1123_date/1 (httpd_util.erl, line 334)

To alleviate this, simply add a clause to handle the empty list and return the
original time in the expected rfc1123 format. This is the approach of other
modules which make use of the calender:local_time_to_universal_time_dst/1
function. The formatted date is then returned without error:

2> httpd_util:rfc1123_date({{2017, 03, 26},{1, 0, 0}}).
   "Sun, 26 Mar 2017 01:00:00 GMT"

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