Make emails serializable (#92)

* Make emails serializable

Fixes <#91>.

* Ensure that an attachment cannot be added more than once to the same email

* Add back `attachments` argument to `.def_equals` call

spec/email_spec.cr

@@ -1,3 +1,4 @@
+require "json"
 require "./spec_helper"
 
 private class User
@@ -84,6 +85,17 @@ private class UndeliverableEmail < Carbon::Email
   to Carbon::Address.new("[email protected]")
 end
 
+private class SerializableEmail < BareMinimumEmail
+  include JSON::Serializable
+
+  MEMORY = IO::Memory.new
+
+  attachment({io: MEMORY, file_name: "file.txt", mime_type: "text/plain"})
+
+  # This is to check that an attachment cannot be added more than once
+  attachment({file_name: "file.txt", mime_type: "text/plain", io: MEMORY})
+end
+
 describe Carbon::Email do
   it "can build a bare minimum email" do
     email = BareMinimumEmail.new
@@ -153,6 +165,13 @@ describe Carbon::Email do
     email.html_body.should contain "Email body"
   end
 
+  it "can be serialized" do
+    email = SerializableEmail.from_json("{}")
+
+    email.attachments.size.should eq(1)
+    email.attachments.first[:io]?.should eq(SerializableEmail::MEMORY)
+  end
+
   context "deliverable?" do
     it "is not delivery it is digiorno" do
       email = UndeliverableEmail.new

src/carbon/email.cr

@@ -91,15 +91,17 @@ abstract class Carbon::Email
     end
   end
 
-  @attachments = [] of Carbon::Attachment
-  getter attachments
+  def attachments
+    [] of Carbon::Attachment
+  end
 
   macro attachment(value)
     def attachments : Array(Carbon::Attachment)
-      {% if @type.methods.map(&.name).includes?("attachments".id) %}
-        previous_def
+      {% if @type.methods.map(&.name).includes?(:attachments.id) %}
+        previous_def | [{{ value }}]
+      {% else %}
+        super | [{{ value }}]
       {% end %}
-      @attachments << {{value}}
     end
   end