display: fixed bug for unsupported tags

Fix crash on unsupported languages, scripts and regions when passed
to the respective Dictionary Namers. Added tests to test passing
unsupported values.

Dropped the uniqueness test. A Namer may now map to its parent's
namer, causing it to no longer be unique.

Change-Id: I1545956a5d40059f5a3fc06128f3e6ee7091d9ef
Reviewed-on: https://go-review.googlesource.com/12903
Reviewed-by: Nigel Tao <nigeltao@golang.org>
3 files changed