Avoid an exception from checking if a string contains a non-string.

lib/liquid/condition.rb

@@ -16,7 +16,12 @@ module Liquid
       '>='.freeze => :>=,
       '<='.freeze => :<=,
       'contains'.freeze => lambda do |cond, left, right|
-        left && right && left.respond_to?(:include?) ? left.include?(right) : false
+        if left && right && left.respond_to?(:include?)
+          right = right.to_s if left.is_a?(String)
+          left.include?(right)
+        else
+          false
+        end
       end
     }
 

test/unit/condition_unit_test.rb

@@ -85,6 +85,11 @@ class ConditionUnitTest < Minitest::Test
     assert_evalutes_false 1, 'contains', 0
   end
 
+  def test_contains_with_string_left_operand_coerces_right_operand_to_string
+    assert_evalutes_true ' 1 ', 'contains', 1
+    assert_evalutes_false ' 1 ', 'contains', 2
+  end
+
   def test_or_condition
     condition = Condition.new(1, '==', 2)