TermQuery.toString() doesn't play nicely with whitespace [LUCENE-4673]

A TermQuery where term.text() contains whitespace outputs incorrect string representation: field:foo bar instead of field:"foo bar". A "correct" representation is such that could be parsed again to the correct Query object (using the correct analyzer, yes, but still).

This may not be so critical, but in our system we use Lucene's QP to parse and then pre-process and optimize user queries. To do that we use Query.toString on some clauses to rebuild the query string.

This can be easily resolved by always adding quote marks before and after the term text in TermQuery.toString. Testing to see if they are required or not is too much work and TermQuery is ignorant of quote marks anyway.

Some other scenarios which could benefit from this change is places where escaped characters are used, such as URLs.

---
Migrated from [LUCENE-4673](https://issues.apache.org/jira/browse/LUCENE-4673) by Itamar Syn-Hershko, 1 vote

Start at TermQuery.toString() and trace how its output is consumed by the query parser. Reproduce a term containing whitespace or escaped characters, then verify that the resulting string can be parsed back into the equivalent query; done means the representation round-trips correctly.

Written by the indexing model from the issue text.