[RAILS] Fixed an error in import Rake task

When the task was called without arguments, it failed because `import_model_desc` was nil

Author: karmi

elasticsearch-rails/lib/elasticsearch/rails/tasks/import.rb

@@ -22,7 +22,7 @@
   task :import => 'import:model'
 
   namespace :import do
-    desc <<-DESC.gsub(/    /, '')
+    import_model_desc = <<-DESC.gsub(/    /, '')
       Import data from your model (pass name as CLASS environment variable).
 
         $ rake environment elasticsearch:import:model CLASS='MyModel'
@@ -39,6 +39,7 @@
       Pass an ActiveRecord scope to limit the imported records:
         $ rake environment elasticsearch:import:model CLASS='Article' SCOPE='published'
     DESC
+    desc import_model_desc
     task :model do
       if ENV['CLASS'].to_s == ''
         puts '='*90, 'USAGE', '='*90, import_model_desc, ""