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