Source parameter not purged when removed from check

[jaxxstorm]

Description of problem

When adding the source parameter to a check, then removing it later, the source parameter hangs around in the JSON
I expected when refreshing the JSON on the check after removing the source parameter, the source parameter remained. In order to fix this, I had to remove the check entirely

To reproduce, simply define a check:

sensu::check { 'test':
  command => '/bin/true'
  source => 'something'
}

Then update it to remove the source parameter:

sensu::check { 'test':
  command => '/bin/true'
}

Platform and version information

• Your OS: Linux
• Your Ruby version: 2.1.1
• Your version of Puppet: 3.8.7
• Your version of Sensu: 0.25.2

[ttarczynski]

In #602 I'm adding a possibility to purge properties with 'absent':

sensu::check { 'test':
  command => '/bin/true',
  source => 'absent'
}

If you need to purge properties by default, we can change the default values in manifests/check.pp from undef to 'absent'. Then a property removed from sensu::check declaration would be also removed from JSON file.