Entering URL of an asset already in {file_managed} leads to PDO exception in Media 7.x-1.x

The uri column in {file_managed} is defined as unique. In Media 2.x, file_uri_to_object() by default reuses an existing fid, but to not break backwards compatibility, that is not the default in Media 1.x. To avoid a PDO exception, the provider must either implement a validate() method that throws an exception that media_internet then displays as a form_set_error(), forcing the user to enter a unique URL, or else pass TRUE to file_uri_to_object(), allowing the Media browser to simply return the already existing fid.

This patch does the latter.