Fix index-oob panic in Take::bytes (#138)

The panic happens when `inner.bytes()` returns a slice smaller than the
limit.

tests/test_take.rs

@@ -0,0 +1,13 @@
+extern crate bytes;
+
+use bytes::Buf;
+use std::io::Cursor;
+
+#[test]
+fn long_take() {
+    // Tests that take with a size greater than the buffer length will not
+    // overrun the buffer. Regression test for #138.
+    let buf = Cursor::new(b"hello world").take(100);
+    assert_eq!(11, buf.remaining());
+    assert_eq!(b"hello world", buf.bytes());
+}