options: initialize all possible options to improve object shape

Options#merge works by duping-then-filling ivars, but due to not all of them being initialized on object creation, each merge had the potential of adding more object shapes for the same class, which breaks one of the most recent ruby optimizations

this was fixed by caching all possible options names at the class level, and using that as reference in the initalize method to nilify all unreferenced options

lib/httpx/options.rb

@@ -15,6 +15,17 @@ module HTTPX
     CONNECT_TIMEOUT = READ_TIMEOUT = WRITE_TIMEOUT = 60
     REQUEST_TIMEOUT = OPERATION_TIMEOUT = nil
 
+    @options_names = []
+
+    class << self
+      attr_reader :options_names
+
+      def inherited(klass)
+        super
+        klass.instance_variable_set(:@options_names, @options_names.dup)
+      end
+    end
+
     # https://github.com/ruby/resolv/blob/095f1c003f6073730500f02acbdbc55f83d70987/lib/resolv.rb#L408
     ip_address_families = begin
       list = Socket.ip_address_list
@@ -87,6 +98,11 @@ module HTTPX
         super
       end
 
+      def freeze
+        super
+        @options_names.freeze
+      end
+
       def method_added(meth)
         super
 
@@ -95,6 +111,8 @@ module HTTPX
         optname = Regexp.last_match(1).to_sym
 
         attr_reader(optname)
+
+        @options_names << optname
       end
     end
 
@@ -145,8 +163,12 @@ module HTTPX
     # This list of options are enhanced with each loaded plugin, see the plugin docs for details.
     def initialize(options = {})
       defaults = DEFAULT_OPTIONS.merge(options)
+
       defaults.each do |k, v|
-        next if v.nil?
+        if v.nil?
+          instance_variable_set(:"@#{k}", v)
+          next
+        end
 
         option_method_name = :"option_#{k}"
         raise Error, "unknown option: #{k}" unless respond_to?(option_method_name)
@@ -154,6 +176,13 @@ module HTTPX
         value = __send__(option_method_name, v)
         instance_variable_set(:"@#{k}", value)
       end
+
+      self.class.options_names.each do |other_ivar|
+        next if defaults.key?(other_ivar)
+
+        instance_variable_set(:"@#{other_ivar}", nil)
+      end
+
       freeze
     end
 
@@ -306,7 +335,11 @@ module HTTPX
 
     def to_hash
       instance_variables.each_with_object({}) do |ivar, hs|
-        hs[ivar[1..-1].to_sym] = instance_variable_get(ivar)
+        val = instance_variable_get(ivar)
+
+        next if val.nil?
+
+        hs[ivar[1..-1].to_sym] = val
       end
     end
 

sig/options.rbs

@@ -2,6 +2,8 @@ module HTTPX
   class Options
     # include _ToHash
 
+    attr_reader self.options_names: Array[Symbol]
+
     BUFFER_SIZE: Integer
     WINDOW_SIZE: Integer
     MAX_BODY_THRESHOLD_SIZE: Integer