Fix basic_auth async builder to send capital "Basic"

Closes #468

src/async_impl/request.rs

@@ -170,7 +170,7 @@ impl RequestBuilder {
             Some(password) => format!("{}:{}", username, password),
             None => format!("{}:", username)
         };
-        let header_value = format!("basic {}", encode(&auth));
+        let header_value = format!("Basic {}", encode(&auth));
         self.header(::header::AUTHORIZATION, &*header_value)
     }