Fix initial headers always being an instance of the default header class

lib/httpx/options.rb

@@ -47,13 +47,13 @@ module HTTPX
         write_timeout: WRITE_TIMEOUT,
         request_timeout: REQUEST_TIMEOUT,
       },
+      :headers_class => Class.new(Headers),
       :headers => {},
       :window_size => WINDOW_SIZE,
       :buffer_size => BUFFER_SIZE,
       :body_threshold_size => MAX_BODY_THRESHOLD_SIZE,
       :request_class => Class.new(Request),
       :response_class => Class.new(Response),
-      :headers_class => Class.new(Headers),
       :request_body_class => Class.new(Request::Body),
       :response_body_class => Class.new(Response::Body),
       :connection_class => Class.new(Connection),
@@ -154,7 +154,7 @@ module HTTPX
     end
 
     def option_headers(value)
-      Headers.new(value)
+      headers_class.new(value)
     end
 
     def option_timeout(value)

lib/httpx/request.rb

@@ -61,7 +61,7 @@ module HTTPX
         @uri = origin.merge("#{base_path}#{@uri}")
       end
 
-      @headers = @options.headers_class.new(@options.headers)
+      @headers = @options.headers.dup
       @headers["user-agent"] ||= USER_AGENT
       @headers["accept"]     ||= "*/*"
 

test/options_test.rb

@@ -45,6 +45,17 @@ class OptionsTest < Minitest::Test
     assert opt2.headers.to_a == [%w[accept */*]], "headers are unexpected"
   end
 
+  def test_options_headers_with_instance
+    proc_headers_class = Class.new(HTTPX::Headers) do
+      def initialize(headers = nil)
+        super(headers.transform_values(&:call))
+      end
+    end
+
+    opts = Options.new(headers_class: proc_headers_class, headers: { "x-number" => -> { 1 + 1 } })
+    assert_equal "2", opts.headers["x-number"]
+  end
+
   def test_options_merge_hash
     opts = Options.new(body: "fat")
     merged_opts = opts.merge(body: "thin")