Added test for using MatchAgainst in where clause

[stollr]

I have added a unit test to show that the current MatchAgainst does not work in a where clause. I think the reason is that the DQL parser thinks it is a simple conditional expression and expects an operator after the AGAINST(...) token.

One can workaround this by adding > 0 to the condition:

  SELECT blank 
  FROM DoctrineExtensions\Tests\Entities\Blank AS blank 
- WHERE MATCH(blank.id) AGAINST ('3')
+ WHERE MATCH(blank.id) AGAINST ('3') > 0

I do not know if this should or can be fixed. But if not, it would be good to add a note somewhere. I can do that if I get a hint where to place it.