Remove special case that breaks 'Reply-To' (#13)

* remove special case that breaks 'Reply-To'

Currently if one does `msg.setHeader('Reply-To', 'value')` that will save the value to the existing instance of `Reply-To` in the headers' `this.store`, and then at dump time the value will be called with `.dump()`, which breaks because `'value'` will be a string and won't have a `.dump()` method.

If we just remove this special case then the `Reply-To` header will be added normally.

* Keep 'Reply-To' but use `v` directly

This reverts commit a766a4e and replaces `v.dump()` with just `v` when dumping the `Reply-To` header, because `v` will be a string.

Co-authored-by: fiatjaf <faitjaf@gmail.com>

src/MIMEMessageHeader.js

@@ -42,7 +42,7 @@ export default class MIMEMessageHeader {
         placement: 'header',
         name: 'Reply-To',
         dump: (v) => {
-          return v.dump()
+          return v
         }
       },
       {