file checksum option always uses md5-sums

Checksumming with md5-files doesn't seem the only problem here, the checksum-type "mtime" seems to have a "bug":

On every sync, the mtime of the target file gets changed, if puppet compares the source-file (mtime some years ago) with the target-file (some 5 minutes ago), it has to resync that file. mtime only seems to make sense, if the corresponding time gets set by puppet after the sync, otherwise we shouldn't support mtime as checksum-type, I guess.

Another problem derived from this bugreport is the one luke mentioned as "pretty shure it is a bug". In source:/trunk/lib/puppet/type/pfile/checksum.rb@1760#L240 the _corresponding_ checksum-type should be used, I guess, not a default. But for now I'm to tired to create a patch, or debug any futher, sorry :)

here is my "testcase", with which I debugged the problem:

$ cat site.pp
file { "/home/thorsten/copy": 
        source => "/home/thorsten/huge",
        checksum => "mtime"
}