Pad secret token correctly

After converting base 16 (hex) string of length 16 to integer and then to
base 36 we can end up with a shorter string due to implicit zeroes on
leading positions in the intermediate integer. This ensures
"00000000000000000000000000000000", "ffffffffffffffffffffffffffffffff"
and everything in between result in a string of length 25 after converting to
base 36.

app/models/asciicast.rb

@@ -64,7 +64,7 @@ class Asciicast < ActiveRecord::Base
   end
 
   def self.generate_secret_token
-    SecureRandom.hex.to_i(16).to_s(36)
+    SecureRandom.hex.to_i(16).to_s(36).rjust(25, '0')
   end
 
   def stdout

db/migrate/20141115172153_add_secret_token_to_asciicasts.rb

@@ -3,7 +3,7 @@ class AddSecretTokenToAsciicasts < ActiveRecord::Migration
     add_column :asciicasts, :secret_token, :string
 
     Asciicast.find_each do |asciicast|
-      asciicast.update_attribute(:secret_token, SecureRandom.hex.to_i(16).to_s(36))
+      asciicast.update_attribute(:secret_token, Asciicast.generate_secret_token)
     end
 
     change_column :asciicasts, :secret_token, :string, null: false