RUBY-716 Make sure parallelCollectionScan command can be run on secondaries

Author: estolfo

lib/mongo/functional/read_preference.rb

@@ -45,6 +45,7 @@ module ReadPreference
       'mapreduce',
       'replsetgetstatus',
       'ismaster',
+      'parallelcollectionscan'
     ]
 
     def self.mongos(mode, tag_sets)

test/unit/read_pref_test.rb

@@ -106,4 +106,10 @@ def test_cmd_no_reroute_with_primary_secondary_ok
     assert_equal :primary, ReadPreference::cmd_read_pref(:primary, command)
   end
 
+  def test_parallel_scan_secondary_ok
+    command = BSON::OrderedHash['parallelCollectionScan', 'test-collection',
+                                'numCursors', 3]
+    assert_equal true, ReadPreference::secondary_ok?(command)
+  end
+
 end