Use array type for array like parameters

[cosmo0920]

I tried to use array type instead of string with #split method.

Because traditional 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]

failed at v0.10.

[cosmo0920]

v0.10 is already EOL.

[sonots]

I drop v0.10 support https://github.com/u-ichi/fluent-plugin-mail/commit/f3c64f1fa6589a8712656d10d89e4592a7d72646. I will merge this.

[cosmo0920]

Thank you!