fix(lint): change deny(missing_docs) to only apply for tests

Closes #600

Author: seanmonstar

src/lib.rs

@@ -1,5 +1,5 @@
 #![doc(html_root_url = "https://hyperium.github.io/hyper/")]
-#![deny(missing_docs)]
+#![cfg_attr(test, deny(missing_docs))]
 #![cfg_attr(test, deny(warnings))]
 #![cfg_attr(all(test, feature = "nightly"), feature(test))]