Handle empty include= parameter.

Author: dougo

lib/jsonapi/request_parser.rb

@@ -345,7 +345,7 @@ def parse_include_directives(resource_klass, raw_include)
 
       included_resources = []
       begin
-        included_resources += raw_include.is_a?(Array) ? raw_include : CSV.parse_line(raw_include)
+        included_resources += raw_include.is_a?(Array) ? raw_include : CSV.parse_line(raw_include) || []
       rescue CSV::MalformedCSVError
         fail JSONAPI::Exceptions::InvalidInclude.new(format_key(resource_klass._type), raw_include)
       end

test/unit/jsonapi_request/jsonapi_request_test.rb

@@ -42,6 +42,11 @@ def test_parse_includes_underscored
     assert request.errors.empty?
   end
 
+  def test_parse_blank_includes
+    include_directives = JSONAPI::RequestParser.new.parse_include_directives(nil, '')
+    assert_empty include_directives.model_includes
+  end
+
   def test_parse_dasherized_with_dasherized_include
     params = ActionController::Parameters.new(
       {