add ClientBuilder.default_headers() for wasm32 target (#1084)

examples/form.rs

@@ -1,9 +1,20 @@
+// Short example of a POST request with form data.
+//
+#[cfg(not(target_arch = "wasm32"))]
 #[tokio::main]
 async fn main() {
-    reqwest::Client::new()
+    let response = reqwest::Client::new()
         .post("http://www.baidu.com")
         .form(&[("one", "1")])
         .send()
         .await
-        .unwrap();
+        .expect("send");
+    println!("Response status {}", response.status());
 }
+
+// The [cfg(not(target_arch = "wasm32"))] above prevent building the tokio::main function
+// for wasm32 target, because tokio isn't compatible with wasm32.
+// If you aren't building for wasm32, you don't need that line.
+// The two lines below avoid the "'main' function not found" error when building for wasm32 target.
+#[cfg(target_arch = "wasm32")]
+fn main() {}