wasm: Add request body in the form of Bytes (#696)

* Add body bytes

* Add example and header creation code

examples/wasm_header/index.js

@@ -0,0 +1,9 @@
+const rust = import('./pkg');
+
+rust
+    .then(m => {
+        return m.run().then((data) => {
+            console.log(data);
+        })
+    })
+    .catch(console.error);