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, 12 insertions, 2 deletions

diff --git a/activerecord/test/cases/defaults_test.rb b/activerecord/test/cases/defaults_test.rb
index 1c96aaabe2..f34501b204 100644
--- a/activerecord/test/cases/defaults_test.rb
+++ b/activerecord/test/cases/defaults_test.rb
@@ -109,13 +109,23 @@ if current_adapter?(:Mysql2Adapter)
     if ActiveRecord::Base.connection.version >= "5.6.0"
       test "schema dump datetime includes default expression" do
         output = dump_table_schema("datetime_defaults")
-        assert_match %r/t\.datetime\s+"modified_datetime",\s+default: -> { "CURRENT_TIMESTAMP" }/, output
+        assert_match %r/t\.datetime\s+"modified_datetime",\s+default: -> { "CURRENT_TIMESTAMP(?:\(\))?" }/i, output
+      end
+
+      test "schema dump datetime includes precise default expression" do
+        output = dump_table_schema("datetime_defaults")
+        assert_match %r/t\.datetime\s+"precise_datetime",.+default: -> { "CURRENT_TIMESTAMP\(6\)" }/i, output
       end
     end
 
     test "schema dump timestamp includes default expression" do
       output = dump_table_schema("timestamp_defaults")
-      assert_match %r/t\.timestamp\s+"modified_timestamp",\s+default: -> { "CURRENT_TIMESTAMP" }/, output
+      assert_match %r/t\.timestamp\s+"modified_timestamp",\s+default: -> { "CURRENT_TIMESTAMP(?:\(\))?" }/i, output
+    end
+
+    test "schema dump timestamp includes precise default expression" do
+      output = dump_table_schema("timestamp_defaults")
+      assert_match %r/t\.timestamp\s+"precise_timestamp",.+default: -> { "CURRENT_TIMESTAMP\(6\)" }/i, output
     end
 
     test "schema dump timestamp without default expression" do