Make headers method on WASM client for compatibility with async_impl (#991)

The replace_headers method had to be moved since the async_impl module
is not compiled while compiling to wasm. This caused the replace_headers
method to be unavailable. fast_random had to be excluded from the wasm
build to prevent dead code warnings in the wasm target.

src/lib.rs

@@ -306,6 +306,7 @@ if_hyper! {
 
 if_wasm! {
     mod wasm;
+    mod util;
 
     pub use self::wasm::{multipart, Body, Client, ClientBuilder, Request, RequestBuilder, Response};
 }