Boost perfect phrase matches.

If the user hasn't enclosed anything in quotes then we assume that they
would like a perfect phrase match if one exists.  This patch rescores
the top bunch of results with a copy of the original query as a phrase
query.

Change-Id: I05f7dcf5fac1223336b3191d18dc084b4bad7232
Tests for boosting perfect phrase matches.

Change-Id: If988f967b19ac318d57717da3009fd2209439556
Merge "Tests for boosting perfect phrase matches."
Merge "Boost perfect phrase matches."