make an explicit key set so they're all normalized as symbols

lib/faraday/options.rb

@@ -48,7 +48,7 @@ def merge(value)
 
     # Public
     def fetch(key, *args)
-      return send(key) if keys.include?(key)
+      return send(key) if symbolized_key_set.include?(key.to_sym)
 
       key_setter = "#{key}="
 
@@ -130,6 +130,10 @@ def [](key)
       end
     end
 
+    def symbolized_key_set
+      @symbolized_key_set ||= Set.new(keys.map { |k| k.to_sym })
+    end
+
     def self.inherited(subclass)
       super
       subclass.attribute_options.update(attribute_options)