fix RequestOptions#merge + test for default options

spec/google/apis/options_spec.rb

@@ -37,4 +37,12 @@ RSpec.describe Google::Apis::RequestOptions do
   it 'should allow nil in merge' do
     expect(options.merge(nil)).to be_an_instance_of(Google::Apis::RequestOptions)
   end
+
+  it 'should override default options' do
+    Google::Apis::RequestOptions.default.header = 'Content-Length: 50'
+    opts = Google::Apis::RequestOptions.new
+    opts.header = 'Content-Length: 70'
+    expect(options.merge(opts).header).to eq 'Content-Length: 70' 
+  end
+
 end