Setting ckan.search.solr_allowed_query_parsers in ini results in error

https://github.com/ckan/ckanext-spatial/blob/master/ckanext/spatial/plugin/__init__.py#L165 assumes that `config["ckan.search.solr_allowed_query_parsers"]` contains a list, but if it is set in ini file (and not by the above default value) it contains a string, causing an error. Also if the string already contains the value in `qp`, adding a duplicate is unnecessary.

Open ckanext/spatial/plugin/__init__.py around line 165 and inspect how ckan.search.solr_allowed_query_parsers is read and updated. Verify the behavior with the setting supplied through an ini file, and consider the existing default path; done means an ini-provided value no longer errors and an already-present parser is not duplicated.

Written by the indexing model from the issue text.