added logic for toupper and tolower on MString.

MString.cpp

@@ -9,6 +9,7 @@
 #include <stdlib.h>
 #include <unistd.h>
 #include <fcntl.h>
+#include <cctype>
 
 namespace coreutils {
    
@@ -377,12 +378,16 @@ namespace coreutils {
    
    MString MString::toUpper() {
       MString target;
+      for(int ix = 0; ix < length; ++ix)
+	write(std::toupper(data[ix]));      
       return target;
    }
    
    
    MString MString::toLower() {
       MString target;
+      for(int ix = 0; ix < length; ++ix)
+	write(std::tolower(data[ix]));      
       return target;
    }