Expose Codec via an unstable flag (#49)

Exposes `Codec` using an unstable flag. This is useful for testing.

tests/codec_read.rs

@@ -0,0 +1,48 @@
+#[macro_use]
+extern crate h2_test_support;
+use h2_test_support::*;
+
+#[test]
+fn read_none() {
+    let mut codec = Codec::from(
+        mock_io::Builder::new()
+        .build());
+
+    assert_closed!(codec);
+}
+
+#[test]
+fn read_data_no_padding() {
+    let mut codec = raw_codec! {
+        read => [
+            0, 0, 5, 0, 0, 0, 0, 0, 1,
+            "hello",
+        ];
+    };
+
+    let data = poll_data!(codec);
+    assert_eq!(data.stream_id(), 1);
+    assert_eq!(data.payload(), &b"hello"[..]);
+    assert!(!data.is_end_stream());
+
+    assert_closed!(codec);
+}
+
+#[test]
+fn read_data_padding() {
+    let mut codec = raw_codec! {
+        read => [
+            0, 0, 11, 0, 0x8, 0, 0, 0, 1,
+            5,       // Pad length
+            "hello", // Data
+            "world", // Padding
+        ];
+    };
+
+    let data = poll_data!(codec);
+    assert_eq!(data.stream_id(), 1);
+    assert_eq!(data.payload(), &b"hello"[..]);
+    assert!(!data.is_end_stream());
+
+    assert_closed!(codec);
+}