Raise in LookupContext#formats= on invalid format

This is a developer quality of life improvement, to ensure that unknown
formats aren't assigned (which it would previously accept, but wouldn't
work 100% correctly due to caching).

actionview/lib/action_view/lookup_context.rb

@@ -284,6 +284,11 @@ module ActionView
         values.concat(default_formats) if values.delete "*/*"
         values.uniq!
 
+        invalid_types = (values - Template::Types.symbols)
+        unless invalid_types.empty?
+          raise ArgumentError, "Invalid formats: #{invalid_types.map(&:inspect).join(", ")}"
+        end
+
         if values == [:js]
           values << :html
           @html_fallback_for_js = true

actionview/test/template/lookup_context_test.rb

@@ -80,6 +80,14 @@ class LookupContextTest < ActiveSupport::TestCase
     assert_equal [:js, :html], @lookup_context.formats
   end
 
+  test "raises on invalid format assignment" do
+    ex = assert_raises ArgumentError do
+      @lookup_context.formats = [:html, :javascript]
+    end
+
+    assert_equal "Invalid formats: :javascript", ex.message
+  end
+
   test "provides getters and setters for locale" do
     @lookup_context.locale = :pt
     assert_equal :pt, @lookup_context.locale