add ClientBuilder.default_headers() for wasm32 target

[stevelr]

Added ClientBuilder.default_headers to wasm, with a couple unit tests.
To make the wasm32 unit tests not generate a barrage of errors for non-wasm code, I added some #cfg gates to some tests in the tests/ folder. Cargo doesn't support target-specific examples, so I had to disable default build of two of the examples that depend on tokio::test.
Unfortunately this will require adding one more step to your CI actions:

cargo build --examples --features __examples

To run the wasm unit tests, use

wasm-pack test --firefox --headless

[seanmonstar]

I'm happy to add the general feature, thanks! A few thoughts inline...

[stevelr]

partially addresses issue #657

[stevelr]

There's another possible way to address the example builds ..

If we add async-std to the dev-dependencies, this code compiles with and without target=wasm32

fn main() {
    async_std::task::block_on(async {
        reqwest::Client::new()
            .post("http://www.baidu.com")
            .form(&[("one", "1")])
            .send()
            .await
            .unwrap();
    });
}

We could also add a comment to explain that reqwest compiles fine with wasm, it's just tokio runtime that is incompatible.
Let me know if you'd like me to make this change to the examples

[stevelr]

Updated per your suggestion - examples build automatically now