MT-12537: remove extraneous headers added by ActionMailer

lib/mailtrap/sending/convert.rb

@@ -24,7 +24,7 @@ def from_message(message) # rubocop:disable Metrics/AbcSize, Metrics/MethodLengt
 
         private
 
-        PROCESSED_HEADERS = %w[
+        SPECIAL_HEADERS = %w[
           from
           to
           cc
@@ -35,14 +35,27 @@ def from_message(message) # rubocop:disable Metrics/AbcSize, Metrics/MethodLengt
           contenttype
         ].freeze
 
+        # ActionMailer adds these headers my calling `Mail::Message#encoded`,
+        # as if the message is to be delivered via SMTP.
+        # Since the message will actually be generated on the Mailtrap side from its components,
+        # the headers are redundant and potentially conflicting, so we remove them.
+        ACTIONMAILER_ADDED_HEADERS = %w[
+          contenttransferencoding
+          date
+          messageid
+          mimeversion
+        ]
+
+        HEADERS_TO_REMOVE = SPECIAL_HEADERS+ACTIONMAILER_ADDED_HEADERS
+
         def prepare_addresses(addresses)
           Array(addresses).map { |address| prepare_address(address) }
         end
 
         def prepare_headers(message)
           message
             .header_fields
-            .reject { |header| PROCESSED_HEADERS.include?(header.name.downcase.delete('-')) }
+            .reject { |header| HEADERS_TO_REMOVE.include?(header.name.downcase.delete('-')) }
             .to_h { |header| [header.name, header.value] }
             .compact
         end

spec/mailtrap/sending/convert_spec.rb

@@ -3,6 +3,10 @@
 RSpec.describe Mailtrap::Sending::Convert do
   describe '.from_message' do
     subject(:mail) do
+      # This method is called by ActionMailer before passing the message to delivery,
+      # and populates some redundant headers on the message.
+      # We want to ensure they are removed.
+      message.encoded
       described_class.from_message(message)
     end
 
@@ -97,7 +101,7 @@
           message.text_part = 'Some text'
         end
 
-        it 'only text is present' do
+        specify 'only text is present' do
           expect(mail.text).to eq('Some text')
           expect(mail.html).to be_nil
         end
@@ -108,7 +112,7 @@
           message.html_part = '<div>HTML part</div>'
         end
 
-        it 'only html is present' do
+        specify 'only html is present' do
           expect(mail.text).to be_nil
           expect(mail.html).to eq('<div>HTML part</div>')
         end
@@ -120,7 +124,7 @@
           message.html_part = '<div>HTML part</div>'
         end
 
-        it 'only html is present' do
+        specify 'both parts are present' do
           expect(mail.text).to eq('Some text')
           expect(mail.html).to eq('<div>HTML part</div>')
         end