Try to fix docs.rs generation (#343)

https://github.com/seanmonstar/reqwest/issues/329
https://github.com/onur/docs.rs/issues/23#issuecomment-419623139

Cargo.toml

@@ -40,3 +40,6 @@ serde_derive = "1.0"
 [features]
 default = []
 hyper-011 = ["hyper-old-types"]
+
+[package.metadata.docs.rs]
+rustdoc-args = ["--cfg", "docs_rs_workaround"]

src/lib.rs

@@ -1,6 +1,7 @@
 #![deny(warnings)]
 #![deny(missing_docs)]
 #![deny(missing_debug_implementations)]
+#![cfg_attr(docs_rs_workaround, feature(extern_prelude))]
 #![doc(html_root_url = "https://docs.rs/reqwest/0.8.6")]
 
 //! # reqwest