`valid_type?` should accept only supported types

`valid_type?` is used in schema dumper to determine if a type is
supported. So if `valid_type?(:foobar)` is true, it means that schema
dumper is allowed to create `t.foobar`. But it doesn't work. I think
that `valid_type?` should accept only supported types.

https://github.com/rails/rails/blob/v5.1.0.beta1/activerecord/lib/active_record/schema_dumper.rb#L135-L142

```ruby
  columns.each do |column|
    raise StandardError, "Unknown type '#{column.sql_type}' for column '#{column.name}'" unless @connection.valid_type?(column.type)
    next if column.name == pk
    type, colspec = @connection.column_spec(column)
    tbl.print "    t.#{type} #{column.name.inspect}"
    tbl.print ", #{format_colspec(colspec)}" if colspec.present?
    tbl.puts
  end
```

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