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

* Add body bytes

* Add example and header creation code

examples/wasm_header/src/lib.rs

@@ -0,0 +1,23 @@
+use wasm_bindgen::prelude::*;
+
+// NOTE: This test is a clone of https://github.com/rustwasm/wasm-bindgen/blob/master/examples/fetch/src/lib.rs
+// but uses Reqwest instead of the web_sys fetch api directly
+
+/**
+* curl --location --request POST "https://postman-echo.com/post" \
+ --data "This is expected to be sent back as part of response body."
+*/
+#[wasm_bindgen]
+pub async fn run() -> Result<JsValue, JsValue> {
+    let res = reqwest::Client::new()
+        .post("https://postman-echo.com/post")
+        .body("This is expected to be sent back as part of response body.")
+        .header("Content-Type", "application/x-www-form-urlencoded")
+        // .header("Access-Control-Allow-Origin", "*")
+        .send()
+        .await?;
+
+    let text = res.text().await?;
+
+    Ok(JsValue::from_str(&text))
+}