Bug fix for ticket #29138

[Constantino Schillebeeckx]

I've put together a ​bug fix for the case when autocomplete_fields is used on a foreign-key field that has on_fields specified; bug originally reported here.

Note I've been able to update the current unit tests to work with my new code; however one of the new unit tests doesn't pass. The error is a bit beyond my knowledge of how Django works and I would appreciate some help in fixing it:

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ERROR: test_success (admin_views.test_autocomplete_view.AutocompleteJsonViewTests)
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Traceback (most recent call last):
  File "/usr/local/opt/pyenv/versions/3.6.3/lib/python3.6/unittest/case.py", line 59, in testPartExecutor
    yield
  File "/usr/local/opt/pyenv/versions/3.6.3/lib/python3.6/unittest/case.py", line 605, in run
    testMethod()
  File "/Users/constantino/Desktop/django/tests/admin_views/test_autocomplete_view.py", line 72, in test_success
    response = AutocompleteJsonView.as_view(**self.as_view_args_to_field)(request)
  File "/Users/constantino/Desktop/django/django/views/generic/base.py", line 68, in view
    return self.dispatch(request, *args, **kwargs)
  File "/Users/constantino/Desktop/django/django/views/generic/base.py", line 88, in dispatch
    return handler(request, *args, **kwargs)
  File "/Users/constantino/Desktop/django/django/contrib/admin/views/autocomplete.py", line 31, in get
    self.object_list = self.get_queryset()
  File "/Users/constantino/Desktop/django/django/contrib/admin/views/autocomplete.py", line 48, in get_queryset
    qs, search_use_distinct = self.model_admin.get_search_results(self.request, qs, self.term)
  File "/Users/constantino/Desktop/django/django/contrib/admin/options.py", line 1026, in get_search_results
    queryset = queryset.filter(reduce(operator.or_, or_queries))
  File "/Users/constantino/Desktop/django/django/db/models/query.py", line 844, in filter
    return self._filter_or_exclude(False, *args, **kwargs)
  File "/Users/constantino/Desktop/django/django/db/models/query.py", line 862, in _filter_or_exclude
    clone.query.add_q(Q(*args, **kwargs))
  File "/Users/constantino/Desktop/django/django/db/models/sql/query.py", line 1263, in add_q
    clause, _ = self._add_q(q_object, self.used_aliases)
  File "/Users/constantino/Desktop/django/django/db/models/sql/query.py", line 1281, in _add_q
    current_negated, allow_joins, split_subq)
  File "/Users/constantino/Desktop/django/django/db/models/sql/query.py", line 1287, in _add_q
    split_subq=split_subq,
  File "/Users/constantino/Desktop/django/django/db/models/sql/query.py", line 1225, in build_filter
    condition = self.build_lookup(lookups, col, value)
  File "/Users/constantino/Desktop/django/django/db/models/sql/query.py", line 1087, in build_lookup
    raise FieldError('Related Field got invalid lookup: {}'.format(lookup_name))
django.core.exceptions.FieldError: Related Field got invalid lookup: icontains