[GH-275] accept array as parameter in getConcatExpression

This issue is created automatically through a Github pull request on behalf of broncha:

Url: https://github.com/doctrine/dbal/pull/275

Message:

The function `getConcatExpression` accepts strings, but as there can be any number of parameters passed to this and while calling from `ConcatFunction` sqlwalker has to walk all the parameters. So it would be better if `getConcatExpression` accepts array. Infact even better if it only accepts array. It would be much cleaner. Also I am opening a pull request in ORM for `ConcatFunction` as it only validates 2 primaryStrings. It should support more than two. SO for that to happen, `getConcatExpression` should accept array as parameter.