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/sendinblue.py

@@ -3,7 +3,7 @@ from requests.structures import CaseInsensitiveDict
 from .base_requests import AnymailRequestsBackend, RequestsPayload
 from ..exceptions import AnymailRequestsAPIError
 from ..message import AnymailRecipientStatus
-from ..utils import get_anymail_setting, parse_address_list
+from ..utils import get_anymail_setting
 
 
 class EmailBackend(AnymailRequestsBackend):
@@ -88,15 +88,8 @@ class SendinBluePayload(RequestsPayload):
     def serialize_data(self):
         """Performs any necessary serialization on self.data, and returns the result."""
 
-        headers = self.data["headers"]
-        if "Reply-To" in headers:
-            # Reply-To must be in its own param
-            reply_to = headers.pop('Reply-To')
-            self.set_reply_to(parse_address_list([reply_to]))
-        if len(headers) > 0:
-            self.data["headers"] = dict(headers)  # flatten to normal dict for json serialization
-        else:
-            del self.data["headers"]  # don't send empty headers
+        if not self.data['headers']:
+            del self.data['headers']  # don't send empty headers
 
         # SendinBlue use different argument's name if we use template functionality
         if self.template_id:
@@ -179,8 +172,7 @@ class SendinBluePayload(RequestsPayload):
             self.data['replyTo'] = self.email_object(emails[0])
 
     def set_extra_headers(self, headers):
-        for key in headers.keys():
-            self.data['headers'][key] = headers[key]
+        self.data['headers'].update(headers)
 
     def set_tags(self, tags):
         if len(tags) > 0: