Revert signature of eq_ignore_ascii_case() to original

Since the methods on u8 directly will shadow the AsciiExt methods,
we cannot change the signature without breaking everything. It
would have been nice to take `u8` as argument instead of `&u8`, but
we cannot break stuff! So this commit reverts it to the original
`&u8` version.

src/libstd/ascii.rs

@@ -685,7 +685,7 @@ impl AsciiExt for [u8] {
     #[inline]
     fn eq_ignore_ascii_case(&self, other: &[u8]) -> bool {
         self.len() == other.len() &&
-        self.iter().zip(other).all(|(a, &b)| {
+        self.iter().zip(other).all(|(a, b)| {
             a.eq_ignore_ascii_case(b)
         })
     }