move typed headers and implement FromRequest (#2094)

Co-authored-by: Rob Ede <robjtede@icloud.com>

src/http/header/if_modified_since.rs

@@ -0,0 +1,41 @@
+use super::{HttpDate, IF_MODIFIED_SINCE};
+
+crate::header! {
+    /// `If-Modified-Since` header, defined in
+    /// [RFC7232](http://tools.ietf.org/html/rfc7232#section-3.3)
+    ///
+    /// The `If-Modified-Since` header field makes a GET or HEAD request
+    /// method conditional on the selected representation's modification date
+    /// being more recent than the date provided in the field-value.
+    /// Transfer of the selected representation's data is avoided if that
+    /// data has not changed.
+    ///
+    /// # ABNF
+    ///
+    /// ```text
+    /// If-Unmodified-Since = HTTP-date
+    /// ```
+    ///
+    /// # Example values
+    /// * `Sat, 29 Oct 1994 19:43:31 GMT`
+    ///
+    /// # Example
+    ///
+    /// ```
+    /// use std::time::{SystemTime, Duration};
+    /// use actix_web::HttpResponse;
+    /// use actix_web::http::header::IfModifiedSince;
+    ///
+    /// let mut builder = HttpResponse::Ok();
+    /// let modified = SystemTime::now() - Duration::from_secs(60 * 60 * 24);
+    /// builder.insert_header(
+    ///     IfModifiedSince(modified.into())
+    /// );
+    /// ```
+    (IfModifiedSince, IF_MODIFIED_SINCE) => [HttpDate]
+
+    test_if_modified_since {
+        // Test case from RFC
+        test_header!(test1, vec![b"Sat, 29 Oct 1994 19:43:31 GMT"]);
+    }
+}