[DSL] Ensure that Collapse class is required

Author: estolfo

lib/elasticsearch/dsl.rb

@@ -22,6 +22,7 @@
 require 'elasticsearch/dsl/search/base_compound_filter_component'
 require 'elasticsearch/dsl/search/base_aggregation_component'
 require 'elasticsearch/dsl/search/query'
+require 'elasticsearch/dsl/search/collapse'
 require 'elasticsearch/dsl/search/filter'
 require 'elasticsearch/dsl/search/aggregation'
 require 'elasticsearch/dsl/search/highlight'