#148: icu_regex_traits::translate_nocase doesn't use case-folding

In translate_nocase replace u_tolower by u_foldCase(c, U_FOLD_CASE_DEFAULT) suitable for single codeunit case folding

include/boost/regex/v4/icu.hpp

@@ -187,7 +187,7 @@ namespace boost {
       }
       char_type translate_nocase(char_type c) const
       {
-         return ::u_tolower(c);
+         return ::u_foldCase(c, U_FOLD_CASE_DEFAULT);
       }
       char_type translate(char_type c, bool icase) const
       {

include/boost/regex/v5/icu.hpp

@@ -161,7 +161,7 @@ public:
    }
    char_type translate_nocase(char_type c) const
    {
-      return ::u_tolower(c);
+      return ::u_foldCase(c, U_FOLD_CASE_DEFAULT);
    }
    char_type translate(char_type c, bool icase) const
    {