FIX: Ensure GridFieldDetailForm has the current record validator.

Currently the validator is only set through
ModelAdmin and not through GridField
directly. This will set the validator based on the
record unless a custom validator has been
provided.

http://www.silverstripe.org/data-model-questions/show/34650?start=8#post331958
Merge pull request #2768 from wilr/gridfieldvalidator

FIX: Ensure GridFieldDetailForm has the current record validator.