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This change fixes the failure of the LibraryTest.filterByPublisher unit-test broken by the previous commit. The previous approach used in `publisherQuery()` for building a phrase query enforcing the specified prefix for all terms fails if 1. the input phrase contains a non-word term that Xapian's query parser doesn't like (e.g. a standalone ampersand character, 1/2, a#1, etc); 2. the input phrase contains at least three terms that Xapian's query parser has no issue with. Using the `quest` tool (coming with xapian-tools under Ubuntu) the issue can be demonstrated as follows: ``` $ quest -o phrase -d some_xapian_db "Energy & security" Parsed Query: Query((energy@1 PHRASE 11 Zsecur@2)) Exactly 0 matches MSet: $ quest -o phrase -d some_xapian_db "Energy & security act" UnimplementedError: OP_NEAR and OP_PHRASE only currently support leaf subqueries $ quest -o phrase -d some_xapian_db 'Energy 1/2 security act' UnimplementedError: OP_NEAR and OP_PHRASE only currently support leaf subqueries $ quest -o phrase -d some_xapian_db "Energy a#1 security act" UnimplementedError: OP_NEAR and OP_PHRASE only currently support leaf subqueries ``` The problem comes from parsing the query with the default operation set to `OP_PHRASE` (exemplified by the `-o phrase` option in above invocations of `quest`). A workaround is to parse the phrase with a default operation of `OP_OR` and then combine all the terms with `OP_PHRASE`. Besides stemming should be disabled in order to target an exact phrase match (save for the non-word terms, if any, that are ignored by the query parser).