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

* Add body bytes

* Add example and header creation code

examples/wasm_header/.gitignore

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+node_modules
+pkg
+target
+Cargo.lock
+*.swp

examples/wasm_header/Cargo.toml

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+[package]
+name = "wasm"
+version = "0.1.0"
+authors = ["John Gallagher <john.willis.gallagher@gmail.com>"]
+edition = "2018"
+
+# Config mostly pulled from: https://github.com/rustwasm/wasm-bindgen/blob/master/examples/fetch/Cargo.toml
+
+[lib]
+crate-type = ["cdylib"]
+
+[dependencies]
+reqwest = {path = "../../"}
+wasm-bindgen-futures = "0.4.1"
+wasm-bindgen = { version = "0.2.51", features = ["serde-serialize"]  }
+

examples/wasm_header/README.md

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+## 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`.

examples/wasm_header/index.js

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

examples/wasm_header/package.json

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+{
+    "scripts": {
+        "build": "webpack",
+        "serve": "webpack-dev-server"
+    },
+    "devDependencies": {
+        "@wasm-tool/wasm-pack-plugin": "1.0.1",
+        "text-encoding": "^0.7.0",
+        "html-webpack-plugin": "^3.2.0",
+        "webpack": "^4.29.4",
+        "webpack-cli": "^3.1.1",
+        "webpack-dev-server": "^3.1.0"
+    }
+}

examples/wasm_header/src/lib.rs

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+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))
+}

examples/wasm_header/webpack.config.js

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+const path = require('path');
+const HtmlWebpackPlugin = require('html-webpack-plugin');
+const webpack = require('webpack');
+const WasmPackPlugin = require("@wasm-tool/wasm-pack-plugin");
+
+module.exports = {
+    entry: './index.js',
+    output: {
+        path: path.resolve(__dirname, 'dist'),
+        filename: 'index.js',
+    },
+    plugins: [
+        new HtmlWebpackPlugin(),
+        new WasmPackPlugin({
+            crateDirectory: path.resolve(__dirname, ".")
+        }),
+        // Have this example work in Edge which doesn't ship `TextEncoder` or
+        // `TextDecoder` at this time.
+        new webpack.ProvidePlugin({
+            TextDecoder: ['text-encoding', 'TextDecoder'],
+            TextEncoder: ['text-encoding', 'TextEncoder']
+        })
+    ],
+    mode: 'development'
+};