for Java6, emulate subset of IETF BCP 47, fixed #474

for Java6, emulate subset of IETF BCP 47, fixed #474
At https://tools.ietf.org/html/bcp47 I read the following paragraph:

 |  Occasionally, the deprecated code is preferred in certain contexts.
 |  For example, both "iw" and "he" can be used in the Java programming
 |  language, but "he" is converted on input to "iw", which is thus the
 |  canonical form in Java.

I think removing `toLowerCase()` is enough here.