Allow "differential.createinline" to accept JSON "false" for "isNewFile"

Summary:
See https://discourse.phabricator-community.org/t/error-creating-inline-comment-via-conduit-api/4535. See T12678.

This API method currently does not accept a JSON "false", but reasonably should.

Test Plan:

• Called method with "isNewFile: false".
  • Before: type error.
  • After: inline comment.

Differential Revision: https://secure.phabricator.com/D21522