Unit test "CHtmlTest" has been updated exposing issue #2423
"CHtml::button()" has been updated to not enforce "value" attribute for the image buttons.
Issue #2423 has been added to CHANGELOG.
Russian guide minor enhancements.
Demo blog, User model enhancements:

1. Added EOL at the end of file.
2. Added @throws tag to the User::generateSalt() method.
3. Removed invalid and unnecessary Yii::t() translation call.
4. Fixes #2375. `str_pad` replaced with `sprintf`.
Merge pull request #2425 from klimov-paul/2423-image-button-value

Fixed #2423: CHtml::button() enforces "value" attribute for the image buttons