Fix char_traits<unsigned char> to what looks like correct behavior.

char_traits<unsigned char>::to_char_type(x) used to return 0 no matter
what x was, and likewise to_int_type(x) used to return 0 no matter what.
(0 is what you get when you default-construct an integral type, which
is what the old code was doing.) This seemed like buggy behavior to me,
so I've changed it.

src/blob.hpp

@@ -96,10 +96,10 @@ namespace std
 
       static char_type
       to_char_type(const int_type& __c)
-      { return char_type(); }
+      { return char_type(__c); }
 
       static int_type
-      to_int_type(const char_type& __c) { return int_type(); }
+      to_int_type(const char_type& __c) { return int_type(__c); }
 
       static bool
       eq_int_type(const int_type& __c1, const int_type& __c2)