I figure out the first question.

[root@mars mysql]# irb
irb(main):001:0> s = String.new
=> ""
irb(main):002:0> s.encoding
=> #<Encoding:ASCII-8BIT>
irb(main):003:0> puts Encoding.default_external.name
UTF-8

Ruby will always use ASCII-8BIT as encoding when you use String.new to 
create a new String object.
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