Fix issue 12455 [reg]Bad lowercase mapping for 'LATIN CAPITAL LETTER I WITH DOT ABOVE'

Also as part of a fix restores a test case in string.d
to exactly match older behaviour.

Some extended greek is not upper but title case, yet changes on toUpper.
std.json.JSONValue toString is now const
Merge pull request #2292 from markisaa/constToString

Issue 13015 - std.json.JSONValue's toString is now const
Merge pull request #2304 from DmitryOlshansky/issue-12455

[REG]Fix issue 12455 Bad lowercase mapping for 'LATIN CAPITAL LETTER I W...