"encoding" is not a valid column option.

[suunnn]

->addColumn('content', 'string', ['encoding' => 'utf8mb4', 'collation' => 'utf8mb4_unicode_ci'])
在php think migrate:run时
报错：
[RuntimeException]
"encoding" is not a valid column option.

think-migration version: 1.*
thinkphp version: 5.0.14

phinx官方文档上说string有encoding选项
http://docs.phinx.org/en/latest/migrations.html#valid-column-options