Adding AllowPartialSearchResults parameter

Just adding this for completeness. I didn't really test using this so
you can definitely remove this if it's not wanted. This support was
added in ES 6.3.

https://www.elastic.co/blog/this-week-in-elasticsearch-and-apache-lucene-2018-02-05

lib/chewy/search/parameters.rb

@@ -10,7 +10,7 @@ module Search
     # @see Chewy::Search::Request#parameters
     # @see Chewy::Search::Parameters::Storage
     class Parameters
-      QUERY_STRING_PARAMS = %i[search_type request_cache].freeze
+      QUERY_STRING_PARAMS = %i[search_type request_cache allow_partial_search_results].freeze
       # Default storage classes warehouse. It is probably possible to
       # add your own classes here if necessary, but I'm not sure it will work.
       #

lib/chewy/search/parameters/allow_partial_search_results.rb

@@ -0,0 +1,27 @@
+require 'chewy/search/parameters/storage'
+
+module Chewy
+  module Search
+    class Parameters
+      # Stores boolean value, but has 3 states: `true`, `false` and `nil`.
+      #
+      # @see Chewy::Search::Request#allow_partial_search_results
+      # @see https://www.elastic.co/guide/en/elasticsearch/reference/6.4/search-request-body.html#_parameters_4
+      class AllowPartialSearchResults < Storage
+        # We don't want to render `nil`, but render `true` and `false` values.
+        #
+        # @see Chewy::Search::Parameters::Storage#render
+        # @return [{Symbol => Object}, nil]
+        def render
+          {self.class.param_name => value} unless value.nil?
+        end
+
+      private
+
+        def normalize(value)
+          !!value unless value.nil?
+        end
+      end
+    end
+  end
+end

spec/chewy/search/parameters_spec.rb

@@ -112,6 +112,11 @@ def storage_object_ids(params)
       specify { expect(subject.render).to eq(search_type: 'query_then_fetch') }
     end
 
+    context do
+      subject { described_class.new(allow_partial_search_results: true) }
+      specify { expect(subject.render).to eq(allow_partial_search_results: true) }
+    end
+
     context do
       subject { described_class.new(query: {foo: 'bar'}, filter: {moo: 'baz'}) }
       specify { expect(subject.render).to eq(body: {query: {bool: {must: {foo: 'bar'}, filter: {moo: 'baz'}}}}) }