Fix #256 -- Allow exact filtering of voucher tags

src/pretix/control/forms/filter.py

@@ -1439,6 +1439,8 @@ class VoucherFilterForm(FilterForm):
             s = fdata.get('tag').strip()
             if s == '<>':
                 qs = qs.filter(Q(tag__isnull=True) | Q(tag=''))
+            elif s[0] == '"' and s[-1] == '"':
+                qs = qs.filter(tag__iexact=s[1:-1])
             else:
                 qs = qs.filter(tag__icontains=s)