Ruby 中函数定义中参数使用 key=value 和 key:value 似乎是差不多的，但是其实还是有很大差异的，特别是有不止一个参数时：
[root@master ruby_learning]# cat test.rb
def test(host = HOST, port = PORT, index = 'jobs')
puts [host, port, index].inspect
end
def test2(host = HOST, port = PORT, index: 'jobs')
puts [host, port, index].inspect
end
HOST='localhost'
PORT='8200'
test(index='nice')
test(HOST, PORT, index='nice')
puts
test2(index:'nice')
test2(HOST, PORT, index:'nice')
[root@master ruby_learning]# ruby test.rb
["nice", "8200", "jobs"]
["localhost", "8200", "nice"]
["localhost", "8200", "nice"]
["localhost", "8200", "nice"]