Fix puppet apply writing source to file when using non-md5 checksum type

puppet apply with

file { "/stuff.txt":

  ensure => file,

  purge => true,

  checksum => sha256,

  source => "/foo.txt",

}

always updates with the notice
Notice: /Stage[main]/Main/File[/stuff.txt]/content: content changed '

36a34910c3f1f68c7fb53a8196c58de86d665c7b84d23a5b3052bca89bc28b1b' to '

fe2df48f1f99a235ee97f2a5e0c3a464'