impl ExactSizeIterator for Iter<T: Buf> (#127)

tests/test_iter.rs

@@ -0,0 +1,22 @@
+extern crate bytes;
+
+use bytes::{Buf, IntoBuf, Bytes};
+
+#[test]
+fn iter_len() {
+    let buf = Bytes::from(&b"hello world"[..]).into_buf();
+    let iter = buf.iter();
+
+    assert_eq!(iter.size_hint(), (11, Some(11)));
+    assert_eq!(iter.len(), 11);
+}
+
+
+#[test]
+fn empty_iter_len() {
+    let buf = Bytes::from(&b""[..]).into_buf();
+    let iter = buf.iter();
+
+    assert_eq!(iter.size_hint(), (0, Some(0)));
+    assert_eq!(iter.len(), 0);
+}