Propel2-619 Propel Query limit(0) not working

changed default value for limit to -1 in order to allow limit clause with 0.
Fixed some unit tests which failed because they expected a default value for limit with 0. Changed this to -1 because otherwise we can't use limit(0) clause.

fixes #619
Merge pull request #701 from nymo/Propel2-619

Propel2-619 Propel Query limit(0) not working