fix MIME generation error with several recipients

Previously a semicolon was used to separate
different addresses.
Now the correct comma separator
is used instead.

lib/mail_address.dart

@@ -43,13 +43,13 @@ class MailAddress extends OnDemandSerializable {
   }
 
   String encode() {
-    if (personalName == null) {
+    final pName = personalName;
+    if (pName == null) {
       return email;
     }
     var buffer = StringBuffer()
       ..write('"')
-      ..write(MailCodec.quotedPrintable
-          .encodeHeader(personalName!, fromStart: true))
+      ..write(MailCodec.quotedPrintable.encodeHeader(pName, fromStart: true))
       ..write('" <')
       ..write(email)
       ..write('>');

lib/message_builder.dart

@@ -405,7 +405,7 @@ class PartBuilder {
 
   /// Adds the header with the specified [name] with the given mail [addresses] as its value
   void setMailAddressHeader(String name, List<MailAddress> addresses) {
-    setHeader(name, addresses.map((a) => a.encode()).join('; '));
+    setHeader(name, addresses.map((a) => a.encode()).join(', '));
   }
 
   void _buildPart() {