BUG #6230: strange changes in behavior of string functions

The following bug has been logged online:

Bug reference:      6230
Logged by:          Dmitry Ryabov
Email address:      dmitry-ryabov@mail.ru
PostgreSQL version: 9.1
Operating system:   Windows x64
Description:        strange changes in behavior of string functions
Details:

In the version 9.1 I was surprised to find that the function "upper" now is
not working as before. For example:

select upper ('SISTEM AYGıTLARı') = 'SISTEM AYGITLARI'

in postgresql 9.0 returns true, but in postgresql 9.1 - false. Such behavior
now occurs under the transformations of other national symbols. In the
release notes that I have not found mention of such changes. Question: is
the bug? Sorry for my bad English.