Enabled ASCIIFolding in Elasticsearch backend

See: http://www.elasticsearch.org/guide/en/elasticsearch/reference/current/analysis-asciifolding-tokenfilter.html

This tells Elasticsearch to convert all characters which are not in the first 127 ASCII characters into their ASCII equivalents. For example: é becomes e and ß becomes ss.

This is already available on fields that are not using partial matching (as it is enabled in Elasticsearch by default). When implementing the partial match analyser I forgot to add this as a filter.