Validate Newsletter segment_recipient value

Why:

A Newsletter can only be sent if the are available user recipient emails
and that means the `segment_recipient` value actually corresponds to a
function on the UserSegments class.

We could rely on the UserSegments::SEGMENTS constant as the list of
possible user segments functions that a Newsletter can use to gather
emails, so any value not included in that hash would not be valid.

But to be 100% sure the newsletter can get a recipients_list we should
just check if the UserSegments class has a method with same name as the
`segment_recipient` value.

How:

* Adding an validation method that checks if UserSegment has a method
with same name as the `segment_recipient` value.
* Adding an scenario to the Newsletter model spec to check this

app/models/newsletter.rb

@@ -4,14 +4,25 @@ class Newsletter < ActiveRecord::Base
   validates :segment_recipient, presence: true
   validates :from, presence: true
   validates :body, presence: true
+  validate :validate_segment_recipient
 
   validates_format_of :from, :with => /@/
 
   def list_of_recipients
-    UserSegments.send(segment_recipient).newsletter
+    UserSegments.send(segment_recipient).newsletter if valid_segment_recipient?
+  end
+
+  def valid_segment_recipient?
+    segment_recipient && UserSegments.respond_to?(segment_recipient)
   end
 
   def draft?
     sent_at.nil?
   end
+
+  private
+
+  def validate_segment_recipient
+    errors.add(:segment_recipient, :invalid) unless valid_segment_recipient?
+  end
 end

config/locales/en/activerecord.yml

@@ -261,6 +261,10 @@ en:
             attachment:
               min_image_width: "Image Width must be at least %{required_min_width}px"
               min_image_height: "Image Height must be at least %{required_min_height}px"
+        newsletter:
+          attributes:
+            segment_recipient:
+              invalid: "The user recipients segment is invalid"
         map_location:
           attributes:
             map:

config/locales/es/activerecord.yml

@@ -257,6 +257,10 @@ es:
             attachment:
               min_image_width: "La imagen debe tener al menos %{required_min_width}px de largo"
               min_image_height: "La imagen debe tener al menos %{required_min_height}px de alto"
+        newsletter:
+          attributes:
+            segment_recipient:
+              invalid: "El segmento de usuarios es inválido"
         map_location:
           attributes:
             map: