Don't raise if no variable found when using context.key?

Previously if you set `strict_variables` to `true` on the context using
`key?('key_name')` would raise a `Liquid::UndefinedVariable` error.
Raising this error makes sense if you're trying to access the variable
directly with something like  `context['key_name']` but by using `key?`
you're safely checking if it exists first.

You should be able to enable `strict_variables` and use `key?` in
combination with each other to ensure code safety.

lib/liquid/context.rb

@@ -184,7 +184,7 @@ module Liquid
     end
 
     def key?(key)
-      self[key] != nil
+      find_variable(key, raise_on_not_found: false) != nil
     end
 
     def evaluate(object)

test/integration/context_test.rb

@@ -639,6 +639,21 @@ class ContextTest < Minitest::Test
     end
   end
 
+  def test_key_lookup_will_raise_for_missing_keys_when_strict_variables_is_enabled
+    context = Context.new
+    context.strict_variables = true
+    assert_raises(Liquid::UndefinedVariable) do
+      context['unknown']
+    end
+  end
+
+  def test_has_key_will_not_raise_for_missing_keys_when_strict_variables_is_enabled
+    context = Context.new
+    context.strict_variables = true
+    refute(context.key?('unknown'))
+    assert_empty(context.errors)
+  end
+
   def test_context_always_uses_static_registers
     registers = {
       my_register: :my_value,