raise ArgumentError if Faraday::Options.memoized is called w/o a block

lib/faraday/options.rb

@@ -172,6 +172,9 @@ module Faraday
     end
 
     def self.memoized(key)
+      if !block_given?
+        raise ArgumentError, "#memoized must be called with a block"
+      end
       memoized_attributes[key.to_sym] = Proc.new
       class_eval <<-RUBY, __FILE__, __LINE__ + 1
         def #{key}() self[:#{key}]; end

spec/faraday/options/options_spec.rb

@@ -250,6 +250,20 @@ RSpec.describe Faraday::Options do
       end
     end
 
+    describe '#memoized' do
+      subject(:options_class) { Class.new(ParentOptions) }
+      it 'requires block' do
+        expect {
+          options_class.memoized(:a)
+        }.to raise_error(ArgumentError)
+      end
+
+      it 'accepts block' do
+        options_class.memoized(:a) { :foo }
+        expect(options_class.new.a).to eql(:foo)
+      end
+    end
+
     describe '#fetch' do
       subject { SubOptions.new }