Xapian queryparser fails when the querystring contains ampersand or other non words

[maneeshpm]

Refer f667d38
We get a xapian exception while trying to parse "Alice & Bob". The error arises because Xapian operators OP_PHRASE and OP_NEAR only support leaf subqueries.

[maneeshpm]

@veloman-yunkan Documenting the following issues about using Xapian Query Parser with OP_PHRASE/OP_NEAR as default operator for future reference:

• Standalone ampersand works fine. "&" gets parsed to "", it gets escaped.
• For some reason, ampersand with exactly two words works fine as well. "Alice & bob" gets parsed to "alice bob".
• As the number of words increases xapian throws an error. "0posanchor Alice & Bob" fails. This also fails at any combination of 3+ words with an ampersand anywhere in between terms or at the beginning. For some reason, it works when the ampersand is at the end.

This was the reason for the failure of the unit test. "Alice & Bob" got parsed till subquery_phrase but failed on subqery_anchored due to the additional anchor term.

As far as fixing this issue is concerned, as you suggested we should use OP_OR as default operator and bind the terms using a query constructor with OP_PHRASE.