support custom collector handler

this allows one to implement any of the curl::easy::Handler
methods that one wish without having to overcomplicate the nice
working out of the box collector provided by your lib

src/collector.rs

@@ -114,6 +114,14 @@ fn send_transfer_info(info: &FileInfo) {
     }
 }
 
+/// This is an extended trait for the curl::easy::Handler trait.
+pub trait ExtendedHandler: Handler {
+    // Return the response body if the Collector if available.
+    fn get_response_body(&self) -> Option<Vec<u8>> {
+        None
+    }
+}
+
 /// The Collector will handle two types in order to store data, via File or via RAM.
 /// Collector::File(FileInfo) is useful to be able to download and upload files.
 /// Collector::Ram(`Vec<u8>`) is used to store response body into Memory.
@@ -188,12 +196,12 @@ impl Handler for Collector {
     }
 }
 
-impl Collector {
+impl ExtendedHandler for Collector {
     /// If Collector::File(FileInfo) is set, there will be no response body since the response
     /// will be stored into a file.
     ///
     /// If Collector::Ram(`Vec<u8>`) is set, the response body can be obtain here.
-    pub fn get_response_body(&self) -> Option<Vec<u8>> {
+    fn get_response_body(&self) -> Option<Vec<u8>> {
         match self {
             Collector::File(_) => None,
             Collector::Ram(container) => Some(container.clone()),

src/error.rs

@@ -1,17 +1,23 @@
 use std::fmt::Debug;
 
-use crate::collector::Collector;
+use crate::collector::ExtendedHandler;
 
 /// Error type returned by failed curl HTTP requests.
 #[derive(Debug)]
-pub enum Error {
+pub enum Error<C>
+where
+    C: ExtendedHandler + Debug + Send + 'static,
+{
     Curl(curl::Error),
     Http(String),
-    Perform(async_curl::error::Error<Collector>),
+    Perform(async_curl::error::Error<C>),
     Other(String),
 }
 
-impl std::fmt::Display for Error {
+impl<C> std::fmt::Display for Error<C>
+where
+    C: ExtendedHandler + Debug + Send + 'static,
+{
     fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
         match self {
             Error::Curl(err) => write!(f, "{}", err),
@@ -22,4 +28,4 @@ impl std::fmt::Display for Error {
     }
 }
 
-impl std::error::Error for Error {}
+impl<C> std::error::Error for Error<C> where C: ExtendedHandler + Debug + Send + 'static {}