For SolrQueryParserBase.getPrefixQuery(String field, String termStr), use BytesRef from calling FieldType.readableToIndexed for Term [LUCENE-5082]

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.

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Migrated from [LUCENE-5082](https://issues.apache.org/jira/browse/LUCENE-5082) by Jessica Cheng Mallet

Start with SolrQueryParserBase.getPrefixQuery and compare it with getFieldQuery and FieldType.getFieldQuery, focusing on readableToIndexed and Term construction. Then locate the relevant parser tests and verify that a custom indexed binary field can process a base64-encoded prefix query through its FieldType; done means the prefix path uses the type-specific BytesRef.

Written by the indexing model from the issue text.