MultiFieldQueryParser does not honor default operator when there is a special character [LUCENE-7597]

simple code snippet

MultiFieldQueryParser mp = new MultiFieldQueryParser(new String[]{"test"}, new StandardAnalyzer());
mp.setDefaultOperator(Operator.AND);
System.out.println(mp.parse(qstring));

if qstring is just 2 tokens, e.g: "you me", behavior is correct:

+(test:you) +(test:me)

now if qstring contains 2 tokens connected by a special char, e.g.: "you&me", behavior is always OR, e.g.

(test:you) (test:me)

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Migrated from [LUCENE-7597](https://issues.apache.org/jira/browse/LUCENE-7597) by John Wang

Start with the MultiFieldQueryParser entry point and reproduce the supplied Java snippet using both space-separated and special-character-separated input. Trace how the default Operator.AND is applied during parsing, and confirm the fix by verifying that the special-character case produces AND-connected clauses like the plain-token case.

Written by the indexing model from the issue text.