Add .remove() to response builder

#705

src/httpresponse.rs

@@ -366,7 +366,7 @@ impl HttpResponseBuilder {
         self
     }
 
-    /// Set a header.
+    /// Append a header.
     ///
     /// ```rust
     /// # extern crate actix_web;
@@ -394,7 +394,7 @@ impl HttpResponseBuilder {
         self
     }
 
-    /// Set a header.
+    /// Append a header.
     ///
     /// ```rust
     /// # extern crate actix_web;
@@ -427,6 +427,33 @@ impl HttpResponseBuilder {
         self
     }
 
+    /// Remove all instances of a header already set on this `HttpResponseBuilder`.
+    ///
+    /// ```rust
+    /// # extern crate actix_web;
+    /// use actix_web::{http, HttpRequest, HttpResponse};
+    ///
+    /// fn index(req: HttpRequest) -> HttpResponse {
+    ///     HttpResponse::Ok()
+    ///         .header(http::header::CONTENT_TYPE, "nevermind") // won't be used
+    ///         .remove(http::header::CONTENT_TYPE)
+    ///         .finish()
+    /// }
+    /// ```
+    pub fn remove<K>(&mut self, key: K) -> &mut Self
+        where HeaderName: HttpTryFrom<K>
+    {
+        if let Some(parts) = parts(&mut self.response, &self.err) {
+            match HeaderName::try_from(key) {
+                Ok(key) => {
+                    parts.headers.remove(key);
+                },
+                Err(e) => self.err = Some(e.into()),
+            };
+        }
+        self
+    }
+
     /// Set the custom reason for the response.
     #[inline]
     pub fn reason(&mut self, reason: &'static str) -> &mut Self {
@@ -1128,6 +1155,31 @@ mod tests {
         assert_eq!(resp.status(), StatusCode::OK);
     }
 
+    #[test]
+    fn test_remove() {
+        let resp = HttpResponse::Ok()
+            .header("deleteme", "value")
+            .remove("deleteme")
+            .finish();
+        assert!(resp.headers().get("deleteme").is_none())
+    }
+
+    #[test]
+    fn test_remove_replace() {
+        let resp = HttpResponse::Ok()
+            .header("some-header", "old_value1")
+            .header("some-header", "old_value2")
+            .remove("some-header")
+            .header("some-header", "new_value1")
+            .header("some-header", "new_value2")
+            .remove("unrelated-header")
+            .finish();
+        let mut v = resp.headers().get_all("some-header").into_iter();
+        assert_eq!("new_value1", v.next().unwrap());
+        assert_eq!("new_value2", v.next().unwrap());
+        assert_eq!(None, v.next());
+    }
+
     #[test]
     fn test_upgrade() {
         let resp = HttpResponse::build(StatusCode::OK).upgrade().finish();