Respect changes in Haml::Options.defaults

in `Haml::TempleEngine` options.

Since the feature has been noted here:
https://github.com/haml/haml/blob/master/REFERENCE.md#options

Note that `Haml::Engine#initialize` in Haml 4 calls
`#initialize_encoding` with the argument `options[:encoding]`, which
should return `nil` instead of `"UTF-8"` (default of `Haml::Options`),
so this is the most backward-compatible behavior.

Fix #1004

lib/haml/engine.rb

@@ -51,6 +51,9 @@ def options_for_buffer
     #   see {file:REFERENCE.md#options the Haml options documentation}
     # @raise [Haml::Error] if there's a Haml syntax error in the template
     def initialize(template, options = {})
+      # Reflect changes of `Haml::Options.defaults` to `Haml::TempleEngine` options, but `#initialize_encoding`
+      # should be run against the arguemnt `options[:encoding]` for backward compatibility with old `Haml::Engine`.
+      options = Options.defaults.dup.tap { |o| o.delete(:encoding) }.merge!(options)
       @options = Options.new(options)
 
       @template = check_haml_encoding(template) do |msg, line|

test/engine_test.rb

@@ -2141,6 +2141,14 @@ def test_unsafe_static_attribute_name_raises_invalid_attribute_name_error
     end
   end
 
+  def test_engine_reflects_defaults
+    defaults = Haml::Options.defaults.dup
+    Haml::Options.defaults[:attr_wrapper] = '"'
+    assert_equal %Q[<a a="a"></a>\n], Haml::Engine.new('%a(a="a")').render
+  ensure
+    Haml::Options.defaults.replace(defaults)
+  end
+
   private
 
   def assert_valid_encoding_comment(comment)