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server: fix crash when Fixnum is sent as a message

lib/tork/server.rb:45:in `block (2 levels) in loop':
undefined method `empty?' for 6:Fixnum (NoMethodError)

lib/tork/cliapp.rb:59:in `recv':
undefined method `lstrip' for 6:Fixnum (NoMethodError)

Apparently, JSON.load() and JSON.parse() are not same:

$ irb -r json
>> JSON.load '6'
6
>> JSON.parse '6'
JSON::ParserError: A JSON text must at least contain two octets!
        from /usr/lib/ruby/2.1.0/json/common.rb:155:in `initialize'
        from /usr/lib/ruby/2.1.0/json/common.rb:155:in `new'
        from /usr/lib/ruby/2.1.0/json/common.rb:155:in `parse'
        from (irb):2
        from /usr/bin/irb:11:in `<main>'