RingBuf: Fix edge cases with mark/reset.

- This fixes #25 and fixes #26.

- Move the mark state into a 'Mark' enum.

- Store both 'pos' and 'len' inside the mark because both
  are needed to distinguish between full/empty states. This
  fixes 'reset' setting 'len' to zero when the buffer was
  full when marked.

- Clear the mark when the length of the marked input range
  exceeds the capacity. Fixes bugs with the mark not being
  cleared correctly on writes.

- Add test cases.

test/test_ring.rs

@@ -89,3 +89,34 @@ fn test_wrap_reset() {
     buf.write(&[1, 2, 3, 4]).unwrap();
     buf.reset();
 }
+
+#[test]
+// Test that writes across a mark/reset are preserved.
+fn test_mark_write() {
+    use std::io::{Read, Write};
+
+    let mut buf = RingBuf::new(8);
+    buf.write(&[1, 2, 3, 4, 5, 6, 7]).unwrap();
+    buf.mark();
+    buf.write(&[8]).unwrap();
+    buf.reset();
+
+    let mut buf2 = [0; 8];
+    buf.read(&mut buf2).unwrap();
+    assert_eq!(buf2, [1, 2, 3, 4, 5, 6, 7, 8]);
+}
+
+#[test]
+// Test that "RingBuf::reset" does not reset the length of a
+// full buffer to zero.
+fn test_reset_full() {
+    use bytes::traits::MutBuf;
+    use std::io::Write;
+
+    let mut buf = RingBuf::new(8);
+    buf.write(&[1, 2, 3, 4, 5, 6, 7, 8]).unwrap();
+    assert_eq!(MutBuf::remaining(&buf), 0);
+    buf.mark();
+    buf.reset();
+    assert_eq!(MutBuf::remaining(&buf), 0);
+}