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/base.py

@@ -4,6 +4,7 @@ import six
 from django.conf import settings
 from django.core.mail.backends.base import BaseEmailBackend
 from django.utils.timezone import is_naive, get_current_timezone, make_aware, utc
+from requests.structures import CaseInsensitiveDict
 
 from ..exceptions import AnymailCancelSend, AnymailError, AnymailUnsupportedFeature, AnymailRecipientsRefused
 from ..message import AnymailStatus
@@ -268,6 +269,8 @@ class BasePayload(object):
                     setter = self.set_html_body if message.content_subtype == 'html' else self.set_text_body
                 elif attr == 'from_email':
                     setter = self.set_from_email_list
+                elif attr == 'extra_headers':
+                    setter = self.process_extra_headers
                 else:
                     # AttributeError here? Your Payload subclass is missing a set_<attr> implementation
                     setter = getattr(self, 'set_%s' % attr)
@@ -278,6 +281,21 @@ class BasePayload(object):
             raise AnymailUnsupportedFeature("%s does not support %s" % (self.esp_name, feature),
                                             email_message=self.message, payload=self, backend=self.backend)
 
+    def process_extra_headers(self, headers):
+        # Handle some special-case headers, and pass the remainder to set_extra_headers.
+        # (Subclasses shouldn't need to override this.)
+        headers = CaseInsensitiveDict(headers)  # email headers are case-insensitive per RFC-822 et seq
+
+        reply_to = headers.pop('Reply-To', None)
+        if reply_to:
+            # message.extra_headers['Reply-To'] will override message.reply_to
+            # (because the extra_headers attr is processed after reply_to).
+            # This matches the behavior of Django's EmailMessage.message().
+            self.set_reply_to(parse_address_list([reply_to]))
+
+        if headers:
+            self.set_extra_headers(headers)
+
     #
     # Attribute validators
     #
@@ -380,6 +398,7 @@ class BasePayload(object):
         self.unsupported_feature('reply_to')
 
     def set_extra_headers(self, headers):
+        # headers is a CaseInsensitiveDict, and is a copy (so is safe to modify)
         self.unsupported_feature('extra_headers')
 
     def set_text_body(self, body):