Feature: Implement merge_headers

Implement and document `merge_headers`
for all other ESPs that can support it. (See #371
for base and Amazon SES implementation.)

Closes #374

tests/test_unisender_go_integration.py

@@ -147,6 +147,18 @@ class UnisenderGoBackendIntegrationTests(AnymailTestMixin, SimpleTestCase):
                 "test+to1@anymail.dev": {"customer-id": "ZXK9123"},
                 "test+to2@anymail.dev": {"customer-id": "ZZT4192"},
             },
+            headers={
+                "List-Unsubscribe-Post": "List-Unsubscribe=One-Click",
+                "List-Unsubscribe": "<mailto:unsubscribe@example.com>",
+            },
+            merge_headers={
+                "test+to1@anymail.dev": {
+                    "List-Unsubscribe": "<https://example.com/a/>",
+                },
+                "test+to2@anymail.dev": {
+                    "List-Unsubscribe": "<https://example.com/b/>",
+                },
+            },
         )
         message.from_email = None  # use template sender
         message.attach("attachment1.txt", "Here is some\ntext", "text/plain")