Fix default value for mysql time types with specified precision

[nkondratyev]

Summary

The TIME, DATETIME, and TIMESTAMP types have supported
a fractional seconds precision from 0 to 6.

Default values from time columns with specified precision is read
as current_timestamp(n) from information schema.

Current behavior

rake db:schema:dump produces schema.rb without default values for time columns with the specified precision:

t.datetime "last_message_at", precision: 6, null: false

Expected behavior

rake db:schema:dump produces schema.rb with default values for time columns with the specified precision:

t.datetime "last_message_at", precision: 6, default: -> { "current_timestamp(6)" }, null: false

[rails-bot]

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[kamipo]

Thanks!