Use whitespace considered split method

[cosmo0920]

This is an another approach of #38. But I dislike this approach because v0.10 is already EOL and fluentd plugin should be encouraged to use convenient method to prevent making a pitfall.

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Because non-regexp splitting way makes some pitfail:

message_out_keys id,name,age

and

message_out_keys id, name, age

is not equal.

Formater is interpreted as ['id', 'name', 'age']. But latter is ['id', ' name', ' age'].

[sonots]

https://github.com/u-ichi/fluent-plugin-mail/pull/38 is merged instead of this.