fix: Add user agent for wasm-pack version check

[drager]

This PR adds a user agent header for the wasm-pack version check. It will look like this: User-Agent: wasm-pack/0.8.1 (https://github.com/rustwasm/wasm-pack).

Fixes #651.

[fitzgen]

Great! Thanks @drager :)

[fitzgen]

It seems like CI has been failing for all the PRs recently and seems sort of related to binary-install. I saw some github emails in that repo, but haven't dug in yet. Do you happen to know what's up?

[drager]

@fitzgen Hmm, I haven't really dug in the latest changes for binary-install just yet. Maybe @ashleygwilliams have an idea?

This is the test that fails though:

wasm-pack/tests/all/build.rs

Lines 24 to 69 in b07ec95

	fn it_should_build_crates_in_a_workspace() {
	let fixture = utils::fixture::Fixture::new();
	fixture
	.file(
	"Cargo.toml",
	r#"
	[workspace]
	members = ["blah"]
	"#,
	)
	.file(
	Path::new("blah").join("Cargo.toml"),
	r#"
	[package]
	authors = ["The wasm-pack developers"]
	description = "so awesome rust+wasm package"
	license = "WTFPL"
	name = "blah"
	repository = "https://github.com/rustwasm/wasm-pack.git"
	version = "0.1.0"
	[lib]
	crate-type = ["cdylib"]
	[dependencies]
	wasm-bindgen = "0.2"
	"#,
	)
	.file(
	Path::new("blah").join("src").join("lib.rs"),
	r#"
	extern crate wasm_bindgen;
	use wasm_bindgen::prelude::*;
	#[wasm_bindgen]
	pub fn hello() -> u32 { 42 }
	"#,
	)
	.install_local_wasm_bindgen();
	fixture
	.wasm_pack()
	.current_dir(&fixture.path.join("blah"))
	.arg("build")
	.assert()
	.success();
	}

[ashleygwilliams]

merging as this change is not related to the CI failure