Fix RequestBuilder::headers to include multiple values

`RequestBuilder::headers` will always overwrite any existing header with
the same name, but will now correctly append extra values from the *new*
header map.

Closes #407

src/async_impl/mod.rs

@@ -7,5 +7,5 @@ pub use self::response::{Response, ResponseBuilderExt};
 pub mod body;
 pub mod client;
 pub mod decoder;
-mod request;
+pub(crate) mod request;
 mod response;