Include output parameter in S3 GetBucketLocation definition

[dtan4]

WHAT

Include output parameter in S3 GetBucketLocation (Aws::S3::Client#get_bucket_location) definition.
This fix enables us to stub response of Aws::S3::Client#get_bucket_location.

WHY

At aws-sdk-core v2.1.14, I cannot stub response of Aws::S3::Client#get_bucket_location.

# test code
      let(:hoge_location) do
        { location_constraint: "ap-northeast-1" }
      end

      before do
        client.stub_responses(:get_bucket_location, hoge_location)
      end

# test result
   Failure/Error: client.stub_responses(:get_bucket_location, hoge_location)
     ArgumentError:
       unexpected value at params[:location_constraint]
     # ./vendor/bundle/ruby/2.2.0/gems/aws-sdk-core-2.1.14/lib/aws-sdk-core/param_validator.rb:26:in `validate!'
...

By inspection on Pry, I found that GetBucketLocation output returns EmptyStructure. I think It may not be expected result...

$ pry -raws-sdk-core
[1] pry(main)> client = Aws::S3::Client.new
=> #<Aws::S3::Client>
[2] pry(main)> client.operation(:get_bucket_location).output
=> #<Seahorse::Model::Shapes::ShapeRef:0x007f88031bb060
 @deprecated=false,
 @metadata={},
 @required=false,
 @shape=
  #<Seahorse::Model::Shapes::StructureShape:0x007f88031bb308
   @members={},
   @members_by_location_name={},
   @metadata={"struct_class"=>Aws::EmptyStructure},
   @name="EmptyStructure",
   @required=#<Set: {}>>>

[trevorrowe]

Thanks!