fixed casting of propertyTask for boolean values

I found this issue while I tried to set the sticky attribute on growlnotify task thru a property def.

Example:
  
   <growlnotify message="test me"  sticky="${stickyOption}" />

To demonstrate this issue I've included a unit test with a adhocTask that simulate the problem I have with growlnotify

Here are the actual results (before this PR)

    PropertyTaskTest > adhoctask:


    PropertyTaskTest > testBooleanValue:

    [boolproperty-demo] Param type is boolean
    [boolproperty-demo] Param type is string

    BUILD FINISHED

And here are the output results it should be expected (after patch )  

    PropertyTaskTest > adhoctask:


    PropertyTaskTest > testBooleanValue:

    [boolproperty-demo] Param type is boolean
    [boolproperty-demo] Param type is boolean

    BUILD FINISHED


Unit test suite provide give us : 

    PHPUnit 3.6.12 by Sebastian Bergmann.

    .

    Time: 4 seconds, Memory: 17.75Mb

    OK (1 test, 2 assertions)


- Laurent