Added FrozenClientRequest for easier retrying HTTP calls (#1064)

* Initial commit

* Added extra_headers

* Added freeze() method to ClientRequest which produces a 'read-only' copy of a request suitable for retrying the send operation

* Additional methods for FrozenClientRequest

* Fix

* Increased crates versions

* Fixed a unit test. Added one more unit test.

* Added RequestHeaderWrapper

* Small fixes

* Renamed RequestHeadWrapper->RequestHeadType

* Updated CHANGES.md files

* Small fix

* Small changes

* Removed *_extra methods from Connection trait

* Added FrozenSendBuilder

* Added FrozenSendBuilder

* Minor fix

* Replaced impl Future with concrete Future implementation

* Small renaming

* Renamed Send->SendBody

actix-http/src/client/error.rs

@@ -128,3 +128,23 @@ impl ResponseError for SendRequestError {
         .into()
     }
 }
+
+/// A set of errors that can occur during freezing a request
+#[derive(Debug, Display, From)]
+pub enum FreezeRequestError {
+    /// Invalid URL
+    #[display(fmt = "Invalid URL: {}", _0)]
+    Url(InvalidUrl),
+    /// Http error
+    #[display(fmt = "{}", _0)]
+    Http(HttpError),
+}
+
+impl From<FreezeRequestError> for SendRequestError {
+    fn from(e: FreezeRequestError) -> Self {
+        match e {
+            FreezeRequestError::Url(e) => e.into(),
+            FreezeRequestError::Http(e) => e.into(),
+        }
+    }
+}