impl: fix cow partialeq impl

Twas a silly mistake. One of the impls was comparing against itself,
which is obviously wrong. We fix it here and add a regression test.
We should probably add tests for other impls as well.

Fixes #82

Author: BurntSushi

src/impls.rs

@@ -33,7 +33,7 @@ macro_rules! impl_partial_eq_cow {
             #[inline]
             fn eq(&self, other: &$lhs) -> bool {
                 let this: &[u8] = (&**other).as_ref();
-                PartialEq::eq(this, other.as_bytes())
+                PartialEq::eq(this, self.as_bytes())
             }
         }
     };
@@ -967,3 +967,18 @@ fn test_debug() {
         B(&format!("{:?}", b"\xFF\xEF\xBF\xBD\xFF".as_bstr())).as_bstr(),
     );
 }
+
+// See: https://github.com/BurntSushi/bstr/issues/82
+#[test]
+fn test_cows_regression() {
+    use crate::ByteSlice;
+    use std::borrow::Cow;
+
+    let c1 = Cow::from(b"hello bstr".as_bstr());
+    let c2 = b"goodbye bstr".as_bstr();
+    assert_ne!(c1, c2);
+
+    let c3 = Cow::from("hello str");
+    let c4 = "goodbye str";
+    assert_ne!(c3, c4);
+}