feat: add method to set raw custom headers in MessageBuilder (#1108)

Closes #661

src/message/mod.rs

@@ -217,7 +217,7 @@ mod mimebody;
 
 use crate::{
     address::Envelope,
-    message::header::{ContentTransferEncoding, Header, Headers, MailboxesHeader},
+    message::header::{ContentTransferEncoding, Header, HeaderValue, Headers, MailboxesHeader},
     Error as EmailError,
 };
 
@@ -369,6 +369,12 @@ impl MessageBuilder {
         self
     }
 
+    /// Set raw custom header to message
+    pub fn raw_header(mut self, raw_header: HeaderValue) -> Self {
+        self.headers.insert_raw(raw_header);
+        self
+    }
+
     /// Add mailbox to header
     pub fn mailbox<H: Header + MailboxesHeader>(self, header: H) -> Self {
         match self.headers.get::<H>() {
@@ -711,7 +717,10 @@ mod test {
             .header(header::To(
                 vec!["Pony O.P. <pony@domain.tld>".parse().unwrap()].into(),
             ))
-            .header(header::Subject::from(String::from("яңа ел белән!")))
+            .raw_header(header::HeaderValue::new(
+                header::HeaderName::new_from_ascii_str("Subject"),
+                "яңа ел белән!".to_owned(),
+            ))
             .body(String::from("Happy new year!"))
             .unwrap();