Fix problem that response format is invalid and cannot get a proper response.

$ curl http://localhost:5000/
curl: (18) transfer closed with outstanding read data remaining

Unless specifying '-E production', Plack::Middleware::Lint says response is invalid.

Plack::Handler::Starlet: Accepting connections at http://0:5000/
Response needs to be 3 element array, or 2 element in streaming at /Users/asato/work/double-boot-problem/local/lib/perl5/Plack/Middleware/Lint.pm line 112
         Plack::Middleware::Lint::validate_res('Plack::Middleware::Lint=HASH(0x7fc3e28a0680)', 'ARRAY(0x7fc3e29494c0)') called at /Users/asato/work/double-boot-problem/local/lib/perl5/Plack/Middleware/Lint.pm line 25
         Plack::Middleware::Lint::call('Plack::Middleware::Lint=HASH(0x7fc3e28a0680)', 'HASH(0x7fc3e2244be0)') called at /Users/asato/work/double-boot-problem/local/lib/perl5/Plack/Component.pm line 50
         Plack::Middleware::StackTrace::try {...}  at /Users/asato/work/double-boot-problem/local/lib/perl5/Try/Tiny.pm line 76
         eval {...} at /Users/asato/work/double-boot-problem/local/lib/perl5/Try/Tiny.pm line 72
         Plack::Middleware::StackTrace::call('Plack::Middleware::StackTrace=HASH(0x7fc3e294c0a8)', 'HASH(0x7fc3e2244be0)') called at /Users/asato/work/double-boot-problem/local/lib/perl5/Plack/Component.pm line 50
         Plack::Component::__ANON__('HASH(0x7fc3e2244be0)') called at /Users/asato/work/double-boot-problem/local/lib/perl5/Plack/Middleware/AccessLog.pm line 24
         Plack::Middleware::AccessLog::call('Plack::Middleware::AccessLog=HASH(0x7fc3e21524b8)', 'HASH(0x7fc3e2244be0)') called at /Users/asato/work/double-boot-problem/local/lib/perl5/Plack/Component.pm line 50
         Plack::Component::__ANON__('HASH(0x7fc3e2244be0)') called at /Users/asato/work/double-boot-problem/local/lib/perl5/Plack/Util.pm line 142
         eval {...} at /Users/asato/work/double-boot-problem/local/lib/perl5/Plack/Util.pm line 142
         Plack::Util::run_app('CODE(0x7fc3e294c270)', 'HASH(0x7fc3e2244be0)') called at /Users/asato/work/double-boot-problem/local/lib/perl5/Starlet/Server.pm line 355
         Starlet::Server::handle_connection('Plack::Handler::Starlet=HASH(0x7fc3e2244478)', 'HASH(0x7fc3e2244be0)', 'IO::Socket::INET=GLOB(0x7fc3e22449b8)', 'CODE(0x7fc3e294c270)', '', '', '') called at /Users/asato/work/double-boot-problem/local/lib/perl5/Starlet/Server.pm line 178
         Starlet::Server::accept_loop('Plack::Handler::Starlet=HASH(0x7fc3e2244478)', 'CODE(0x7fc3e294c270)', 100) called at /Users/asato/work/double-boot-problem/local/lib/perl5/Plack/Handler/Starlet.pm line 75
         Plack::Handler::Starlet::run('Plack::Handler::Starlet=HASH(0x7fc3e2244478)', 'CODE(0x7fc3e294c270)') called at /Users/asato/work/double-boot-problem/local/lib/perl5/Plack/Loader.pm line 84
         Plack::Loader::run('Plack::Loader=HASH(0x7fc3e1828610)', 'Plack::Handler::Starlet=HASH(0x7fc3e2244478)') called at /Users/asato/work/double-boot-problem/local/lib/perl5/Plack/Runner.pm line 277
         Plack::Runner::run('Plack::Runner=HASH(0x7fc3e1827c50)') called at /Users/asato/work/double-boot-problem/local/bin/plackup line 10
Merge pull request #1 from astj/fix_invalid_res

Fix problem that response format is invalid and cannot get a proper response