Whitespace in file uri is urlencoded and breaks file copy with MigrateFileUri

#1718980: Whitespace in file uri throws MigrateException introduced a change to deal with a lack of URL encoding in file URIs, but seems to have broken migrations with MigrateFileFid with source file URIs that include whitespace.

As a demo, update migrate to -dev, and install the migrate_extras_media module. In the source XML doc migrate_extras_media.xml, change the filename from file-ok.png to "file ok.png". Make the same change to the file-ok.png image in the source_files directory, so it matches the XML doc. Then run the migration. It will fail with:

The specified file sites/all/modules/contrib/migrate_extras/migrate_extras_examples/migrate_extras_media/source_files/images/watchdog%20ok.png could not be copied to public://watchdog ok.png.

The same migration works fine with the same data against 2.4.

In my case I'm pulling filenames out of a legacy MySQL DB that has filenames with spaces in them, and they are NOT urlencoded in the source DB. My first step in debugging before I found out this was a change in -dev was to urldecode the filenames in prepareRow, which did not work, because migrate is urlencoding internally, as in the referenced issue. I then verified that none of the source data was urlencoded, so this is definitely internal to migrate.

Perhaps this is specifically an issue only when using MigrateFileFid?