feat(http1): add options to preserve header casing (#2480)

Decouple preserving header case from FFI:

The feature is now supported in both the server and the client
and can be combined with the title case feature, for headers
which don't have entries in the header case map.

Closes #2313

src/ext.rs

@@ -0,0 +1,64 @@
+//! HTTP extensions
+
+use bytes::Bytes;
+#[cfg(feature = "http1")]
+use http::header::{HeaderName, IntoHeaderName, ValueIter};
+use http::HeaderMap;
+
+/// A map from header names to their original casing as received in an HTTP message.
+///
+/// If an HTTP/1 response `res` is parsed on a connection whose option
+/// [`http1_preserve_header_case`] was set to true and the response included
+/// the following headers:
+///
+/// ```ignore
+/// x-Bread: Baguette
+/// X-BREAD: Pain
+/// x-bread: Ficelle
+/// ```
+///
+/// Then `res.extensions().get::<HeaderCaseMap>()` will return a map with:
+///
+/// ```ignore
+/// HeaderCaseMap({
+///     "x-bread": ["x-Bread", "X-BREAD", "x-bread"],
+/// })
+/// ```
+///
+/// [`http1_preserve_header_case`]: /client/struct.Client.html#method.http1_preserve_header_case
+#[derive(Clone, Debug)]
+pub(crate) struct HeaderCaseMap(HeaderMap<Bytes>);
+
+#[cfg(feature = "http1")]
+impl HeaderCaseMap {
+    /// Returns a view of all spellings associated with that header name,
+    /// in the order they were found.
+    pub(crate) fn get_all<'a>(
+        &'a self,
+        name: &HeaderName,
+    ) -> impl Iterator<Item = impl AsRef<[u8]> + 'a> + 'a {
+        self.get_all_internal(name).into_iter()
+    }
+
+    /// Returns a view of all spellings associated with that header name,
+    /// in the order they were found.
+    pub(crate) fn get_all_internal<'a>(&'a self, name: &HeaderName) -> ValueIter<'_, Bytes> {
+        self.0.get_all(name).into_iter()
+    }
+
+    pub(crate) fn default() -> Self {
+        Self(Default::default())
+    }
+
+    #[cfg(any(test, feature = "ffi"))]
+    pub(crate) fn insert(&mut self, name: HeaderName, orig: Bytes) {
+        self.0.insert(name, orig);
+    }
+
+    pub(crate) fn append<N>(&mut self, name: N, orig: Bytes)
+    where
+        N: IntoHeaderName,
+    {
+        self.0.append(name, orig);
+    }
+}