Fix test that didn't compile.

If tomprogrammer#18 (str_into_from) is merged before this pull request, this commit can be omitted.

Author: tormol

src/ascii_str.rs

@@ -417,8 +417,8 @@ mod tests {
         fn generic<C:IntoAsciiStr+?Sized>(c: &C) -> Result<&AsciiStr,IntoAsciiStrError> {
             c.into_ascii()
         }
-        assert_eq!(generic("A"), Ok([Ascii::A].as_ref().into()));
-        assert_eq!(generic(&b"A"[..]), Ok([Ascii::A].as_ref().into()));
+        assert_eq!(generic("A").map(AsciiStr::as_slice), Ok(&[Ascii::A][..]));
+        assert_eq!(generic(&b"A"[..]).map(AsciiStr::as_slice), Ok(&[Ascii::A][..]));
     }
 
     #[test]