Update Windows1251Encoding.cpp

 * Add one missing letter 'я' (0x044f).
 * Add encoding names. According to the document below, the default encoding name should be "windows-1251".
   http://www.iana.org/assignments/character-sets/character-sets.xhtml
Merge pull request #521 from Denisss025/fix_cp1251

Update Windows1251Encoding.cpp