…e agnostic

5 years ago, I made dumping full table options at rails#17569, especially to
dump `ENGINE=InnoDB ROW_FORMAT=DYNAMIC` to use utf8mb4 with large key
prefix.

In that time, omitting the default engine `ENGINE=InnoDB` was not useful
since `ROW_FORMAT=DYNAMIC` always remains as long as using utf8mb4 with
large key prefix.

But now, MySQL 5.7.9 has finally changed the default row format to
DYNAMIC, utf8mb4 with large key prefix can be used without dumping the
default engine and the row format explicitly.

So now is a good time to make the default engine is omitted.

Before:

```ruby
create_table "accounts", options: "ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci", force: :cascade do |t|
end
```

After:

```ruby
create_table "accounts", charset: "utf8mb4", collation: "utf8mb4_0900_ai_ci", force: :cascade do |t|
end
```

To entirely omit `:options` option to make schema agnostic, I've added
`:charset` and `:collation` table options to exclude `CHARSET` and
`COLLATE` from `:options`.

Fixes rails#26209.
Closes rails#29472.

See also rails#33608, rails#33853, and rails#34742.

rails/rails#34742 で不要になった。

Fixes #237

rails/rails#34742 で不要になった。