Add a user-agent header (#605)

src/http.rs

@@ -2,9 +2,15 @@
 //! In the future, if we want to migrate to a different HTTP library,
 //! we can easily change this facade instead of multiple places in the crate.
 
+use reqwest::blocking::Client;
+
 pub type Error = reqwest::Error;
 pub type Response = reqwest::blocking::Response;
 
 pub fn get(url: &str) -> Result<Response, Error> {
-    reqwest::blocking::get(url)
+    Client::new()
+        .get(url)
+        // Some sites require a user agent.
+        .header("User-Agent", concat!("fnm ", env!("CARGO_PKG_VERSION")))
+        .send()
 }