Bug 25587: fix for "clear" button to reset all selects

Button "clear" on cgi-bin/koha/admin/smart-rules.pl did not reset fields
with a dropdown selector except the first one.

This happens because jQuery selector .find("select option:first")
in koha-tmpl/intranet-tmpl/prog/en/modules/admin/smart-rules.tt
selects only single first element in the whole group, as from jquery
doc:
> The :first pseudo-class is equivalent to :eq( 0 ). It could also be
> written as :lt( 1 ). While this matches only a single element,
> :first-child can match more than one: One for each parent.
> (https://api.jquery.com/first-selector/)

And it works if replaced by: .find("select option:first-child")

To reproduce:
    1) Go to /cgi-bin/koha/admin/smart-rules.pl
    2) Start editing one of existing rules or filling a new one, change
       all selects to have non-first option selected. Fill text input
       fields with sample data.
    3) Reset all fields with the "clear" button.
    4) Observe that only the first dropdown field was reset to the first
       option while all others still contain previously given values,
       meanwhile all text input fields are cleared as expected.
    5) Apply the patch.
    6) Repeat steps 1, 2 and 3.
    7) Check that all dropdown fields were reset to first option, as
       expected.

Mentored-by: Andrew Nugged <nugged@gmail.com>

Signed-off-by: Lucas Gass <lucas@bywatersolutions.com>
Signed-off-by: Julian Maurice <julian.maurice@biblibre.com>

Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>

koha-tmpl/intranet-tmpl/prog/en/modules/admin/smart-rules.tt

@@ -941,7 +941,7 @@
                 }
             });
             $(edit_row).find("select").prop('disabled', false);
-            $(edit_row).find("select option:first").attr("selected", "selected");
+            $(edit_row).find("select option:first-child").attr("selected", "selected");
             $(edit_row).find("td:last input[name='clear']").remove();
         }