Bugfix for index_fieldname argument which didn't have behaviour mentioned in the doc

From doc:
> "The index_fieldname option allows you to force the name of the field in the index. This does not change how Haystack refers to the field. This is useful when using Solr’s dynamic attributes or when integrating with other external software.
Default is variable name of the field within the SearchIndex."

However, once index_fieldname is set it was possible to refer to the related field only using its value, but not declared in an index class field name.
This patch fixes this behavior, so that index_fieldname will affect only how indexed field is stored in a backend, while it will be possible to refer to it using declared in the index class field name.