Postmark: handle Reply-To in EmailMessage headers

Move 'Reply-To' header into dedicated Postmark API param

Fixes #39

anymail/backends/postmark.py

@@ -1,5 +1,7 @@
 import re
 
+from requests.structures import CaseInsensitiveDict
+
 from ..exceptions import AnymailRequestsAPIError
 from ..message import AnymailRecipientStatus
 from ..utils import get_anymail_setting
@@ -140,9 +142,12 @@ class PostmarkPayload(RequestsPayload):
             self.data["ReplyTo"] = reply_to
 
     def set_extra_headers(self, headers):
+        header_dict = CaseInsensitiveDict(headers)
+        if 'Reply-To' in header_dict:
+            self.data["ReplyTo"] = header_dict.pop('Reply-To')
         self.data["Headers"] = [
             {"Name": key, "Value": value}
-            for key, value in headers.items()
+            for key, value in header_dict.items()
         ]
 
     def set_text_body(self, body):