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

* Add body bytes

* Add example and header creation code

examples/wasm_header/README.md

@@ -0,0 +1,11 @@
+## Example usage of Reqwest from WASM
+
+You can build the example locally with:
+
+
+    npm run serve
+
+and then visiting http://localhost:8080 in a browser should run the example!
+
+
+This example is loosely based off of [this example](https://github.com/rustwasm/wasm-bindgen/blob/master/examples/fetch/src/lib.rs), an example usage of `fetch` from `wasm-bindgen`.