empty charset fix

lib/mail/header.rb

@@ -169,7 +169,11 @@ def []=(name, value)
     def charset=(val)
       params = self[:content_type].parameters rescue nil
       if params
-        params[:charset] = val
+        if val
+          params[:charset] = val
+        else
+          params.delete(:charset)
+        end
       end
       @charset = val
     end

lib/mail/message.rb

@@ -1477,7 +1477,9 @@ def add_charset
           warning = "Non US-ASCII detected and no charset defined.\nDefaulting to UTF-8, set your own if this is incorrect.\n"
           warn(warning)
         end
-        header[:content_type].parameters['charset'] = @charset
+        if @charset
+          header[:content_type].parameters['charset'] = @charset
+        end
       end
     end
 

spec/mail/message_spec.rb

@@ -1437,6 +1437,39 @@ def message_headers_should_match(message, other)
         expect(mail.parts.last.content_transfer_encoding).to match(/7bit|8bit|binary/)
       end
 
+      describe 'charset=' do
+        before do
+          @mail = Mail.new do
+            to 'mikel@test.lindsaar.net'
+            from 'bob@test.lindsaar.net'
+            subject 'Multipart email'
+            text_part do
+              body 'This is plain text'
+            end
+            html_part do
+              content_type 'text/html; charset=UTF-8'
+              body '<h1>This is HTML</h1>'
+            end
+          end
+        end
+
+        it "should not add an empty charset header" do
+          @mail.charset = nil
+
+          expect(@mail.multipart?).to eq true
+          expect(@mail.parts.count).to eq 2
+          expect(@mail.encoded.scan(/charset=UTF-8/).count).to eq 2
+        end
+
+        it "should remove the charset header" do
+          @mail.charset = 'iso-8859-1'
+          @mail.charset = nil
+
+          expect(@mail.encoded.scan(/charset=UTF-8/).count).to eq 2
+          expect(@mail.encoded.scan(/charset=iso-8859-1/).count).to eq 0
+        end
+      end
+
       describe "convert_to_multipart" do
         subject do
           read_fixture('emails', 'attachment_emails', 'attachment_only_email.eml').tap(&:convert_to_multipart)