Cleanup: centralize Reply-To header handling; case-insensitive headers

Django allows setting the reply address with either message.reply_to
or message.extra_headers["Reply-To"]. If both are supplied, the extra
headers version takes precedence. (See EmailMessage.message().)

Several Anymail backends had duplicate logic to handle conflicting
properties. Move that logic into the base Payload.

(Also prepares for common handling of extra_headers['From'], later.)

Related changes:

* Use CaseInsensitiveDict for processing extra_headers.
  This is potentially a breaking change, but any code that was trying
  to send multiple headers differing only in case was likely already
  broken. (Email header field names are case-insensitive, per RFC-822.)

* Handle CaseInsensitiveDict in RequestsPayload.serialize_json().
  (Several backends had duplicate code for handling this, too.)

* Fixes SparkPost backend, which had been incorrectly treating
  message.reply_to and message.extra_headers['Reply-To'] differently.

anymail/backends/postmark.py

@@ -1,7 +1,5 @@
 import re
 
-from requests.structures import CaseInsensitiveDict
-
 from ..exceptions import AnymailRequestsAPIError
 from ..message import AnymailRecipientStatus
 from ..utils import get_anymail_setting
@@ -149,12 +147,9 @@ 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 header_dict.items()
+            for key, value in headers.items()
         ]
 
     def set_text_body(self, body):