How to hash SHA256 from string?

[zoujiaqing]

```D
import std.stdio;
import std.digest.sha;

void main()
{

     SHA256 sha256;
     sha256.start();
     string appKey = 
"11111111111111111111111111111111111111111111111111111";
     ubyte[1024] data = cast(ubyte[])(appKey.dup[0..$]);
     sha256.put(data);
     ubyte[32] sign = sha256.finish();

     string sign1 = cast(string) sign[0..$];
     writeln("sign: %s", sign1);
}
```

The result varies when you run the code repeatedly and the 
display is garbled:
```
zoujiaqing at mac test % ./test
Getui access sign: %s>tM?a?j,???ߥm?8l~??uzU?|9?~ˡ
zoujiaqing at mac test % ./test
Getui access sign: %s1-??U?
?d<3^3??נ? ??P%u/Iv
zoujiaqing at mac test % ./test
Getui access sign: %s1?ϻN?????ށ?`O?p!?O?4U
:8J~%ʬ
zoujiaqing at mac test % ./test
Getui access sign: %s??????k#O?;?ڋ?5T?"=??;???e
```