For SolrQueryParserBase.getPrefixQuery(String field, String termStr), use BytesRef from calling FieldType.readableToIndexed for Term

Currently, SolrQueryParserBase.getFieldQuery calls FieldType.getFieldQuery, which in turn calls readableToIndexed to get a type-specific BytesRef to pass to Term constructor. I would like SolrQueryParserBase.getPrefixQuery to do the same thing so that I can implement my own indexed binary field and allow my field to process my base64 encoded string query.