istextorbinary returns null for filenames with no extension, and the
isTextFile() guard treated null as "not text" — so terms in LICENSE,
COPYING, AUTHORS, README (extensionless), CHANGELOG, NOTICE, and
similar conventional filenames went through the binary passthrough
in AnonymizeTransformer and were never anonymized.
Add a small whitelist of these names ahead of the istextorbinary call.
Fixes#493.
tdurieux