Fix default value for mysql time types with specified precision

The TIME, DATETIME, and TIMESTAMP types [have supported](https://mariadb.com/kb/en/library/microseconds-in-mariadb/)
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.

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

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

1 files changed, 2 insertions, 0 deletions

diff --git a/activerecord/test/schema/mysql2_specific_schema.rb b/activerecord/test/schema/mysql2_specific_schema.rb
index e634e9e6b1..1ac3b60c92 100644
--- a/activerecord/test/schema/mysql2_specific_schema.rb
+++ b/activerecord/test/schema/mysql2_specific_schema.rb
@@ -5,12 +5,14 @@ ActiveRecord::Schema.define do
   if ActiveRecord::Base.connection.version >= "5.6.0"
     create_table :datetime_defaults, force: true do |t|
       t.datetime :modified_datetime, default: -> { "CURRENT_TIMESTAMP" }
+      t.datetime :precise_datetime, precision: 6, default: -> { "CURRENT_TIMESTAMP(6)" }
     end
   end
 
   create_table :timestamp_defaults, force: true do |t|
     t.timestamp :nullable_timestamp
     t.timestamp :modified_timestamp, default: -> { "CURRENT_TIMESTAMP" }
+    t.timestamp :precise_timestamp, precision: 6, default: -> { "CURRENT_TIMESTAMP(6)" }
   end
 
   create_table :binary_fields, force: true do |t|