multipart upload got 477 using aws-sdk-ruby.

RiakCS respond 477 when I upload file using multipart upload with aws-sdk-ruby.

## to reproduce

```
git checkout bugfix/kaz-update-ruby-test
make test-ruby
```

## stack trace
```
/Users/kazuhiro/.rbenv/versions/1.9.3-p194/lib/ruby/gems/1.9.1/gems/aws-sdk-1.8.5/lib/aws/core/client.rb:339:in `return_or_raise': AWS::Errors::Base
        from /Users/kazuhiro/.rbenv/versions/1.9.3-p194/lib/ruby/gems/1.9.1/gems/aws-sdk-1.8.5/lib/aws/core/client.rb:440:in `client_request'
        from (eval):3:in `complete_multipart_upload'
        from /Users/kazuhiro/.rbenv/versions/1.9.3-p194/lib/ruby/gems/1.9.1/gems/aws-sdk-1.8.5/lib/aws/s3/multipart_upload.rb:244:in `complete'
        from /Users/kazuhiro/.rbenv/versions/1.9.3-p194/lib/ruby/gems/1.9.1/gems/aws-sdk-1.8.5/lib/aws/s3/multipart_upload.rb:267:in `close'
        from /Users/kazuhiro/.rbenv/versions/1.9.3-p194/lib/ruby/gems/1.9.1/gems/aws-sdk-1.8.5/lib/aws/s3/s3_object.rb:713:in `multipart_upload'
        from /Users/kazuhiro/.rbenv/versions/1.9.3-p194/lib/ruby/gems/1.9.1/gems/aws-sdk-1.8.5/lib/aws/s3/s3_object.rb:1629:in `write_with_multipart'
        from /Users/kazuhiro/.rbenv/versions/1.9.3-p194/lib/ruby/gems/1.9.1/gems/aws-sdk-1.8.5/lib/aws/s3/s3_object.rb:604:in `write'
        from hoge.rb:16:in `<main>'
```

## code

```ruby
!#/usr/bin/env ruby
 
require 'aws-sdk'
 
s3client = AWS::S3.new(
  access_key_id:  "your-id",
  secret_access_key: "your-secret-key",
  proxy_uri: "http://localhost:8080",
  use_ssl: false,
  max_retries: 0,
)
bucket = s3client.buckets.create 'simple-bucket-xxxxxxxxxxxxxxxxxxxxxxxx'
bucket.objects['multi'].write(:file => 'Rakefile', :multipart_threshold => 100)
puts bucket.objects['multi'].read
```

- [http trace log](https://gist.github.com/ksauzz/9129d0b3c9aee48172cc#file-error-md)