icontains lookup doesn't work with dictionary params

[yushkovskiy]

ProgrammingError: operator does not exist: text -> unknown
LINE 1: ...act" WHERE (UPPER("app_model"."hstorefield"::text)->'custom_...
^

I believe that problem is in replacement of '::text' to '::hstore'. The condition
https://github.com/djangonauts/django-hstore/blob/master/django_hstore/lookups.py#L82
checks that lhs ends with '::text' cast, but in case of icontains it ends with '::text)' (at least with django 1.7)

python version: 2.7.6
django version: 1.7.10
django-hstore version: 1.4.1