代码：
[1] pry(main)> h = { a: 1, b: 2 }
=> {:a=>1, :b=>2}
[2] pry(main)> hh = { **h, c: 3 }
=> {:a=>1, :b=>2, :c=>3}
[3] pry(main)> hhh = { "a" => 1, "b" => 2 }
=> {"a"=>1, "b"=>2}
[4] pry(main)> hhhh = { **hhh, "c" => 3 }
TypeError: wrong argument type String (expected Symbol)
from (pry):4:in `__pry__'
[5] pry(main)>
ruby 2.x 增加了 keyword arguments, 而 **hash 也只是针对 keyword arguments 的。
http://stackoverflow.com/questions/25191747/why-does-double-splat-only-work-with-symbol-keys
按需取用吧，**的好处是以下情况可以正确捕获
def t a = 1, b, **c
p a,b,c
end
def t2 a = 1, *b, **c
p a,b,c
end
t 1, a: 1 #=> 1 1 {:a=>1}
t 1, 'a' => 1 #=> 1 {"a"=>1} {}
t2 1, a: 1 #=> 1 [] {:a=>1}
t2 1, 'a' => 1 #=> 1 [{"a"=>1}] {}
行为各异，能用就好