Refine code for rust edition 2018

Refine code to support rust edition 2018, no functional changes.

Signed-off-by: Liu Jiang <gerry@linux.alibaba.com>

src/common/headers.rs

@@ -3,7 +3,7 @@
 
 use std::result::Result;
 
-use RequestError;
+use super::RequestError;
 
 /// Wrapper over an HTTP Header type.
 #[derive(Debug, Eq, Hash, PartialEq)]
@@ -58,7 +58,7 @@ impl Header {
 /// invalidate our request as we don't support the full set of HTTP/1.1 specification.
 /// Such header entries are "Transfer-Encoding: identity; q=0", which means a compression
 /// algorithm is applied to the body of the request, or "Expect: 103-checkpoint".
-#[derive(Debug)]
+#[derive(Debug, PartialEq)]
 pub struct Headers {
     /// The `Content-Length` header field tells us how many bytes we need to receive
     /// from the source after the headers.