Docs: Disable default conversion to `timestamptz` on pgloader

What does this MR do?

pgloader default convert MySQL datetime column to PostgreSQL timestamptz column.

c.f. https://pgloader.readthedocs.io/en/latest/ref/mysql.html

type datetime to timestamptz

But this causes unexpected behavior in Rails.

I confirmed with the following version.

• gitlab-ce 10.4.0 (rails 4.2.10)
• pgloader 3.5.10
• MySQL 5.5
• PostgreSQL 10

Expected

Time.zone = "Tokyo"

Time.zone
# => #<ActiveSupport::TimeZone:0x00007f85f15a6710 @name="Tokyo", @utc_offset=nil, @tzinfo=#<TZInfo::TimezoneProxy: Asia/Tokyo>>

Time.current
# => Mon, 29 Jul 2019 16:04:06 JST +09:00

BroadcastMessage
# => BroadcastMessage(id: integer, message: text, starts_at: datetime, ends_at: datetime, created_at: datetime, updated_at: datetime, color: string, font: string, message_html: text, cached_markdown_version: integer)

BroadcastMessage.first.starts_at
# => Wed, 24 Jul 2019 19:00:00 JST +09:00

BroadcastMessage.first.starts_at.class
# => ActiveSupport::TimeWithZone

Actual

Time.zone = "Tokyo"

Time.zone
# => #<ActiveSupport::TimeZone:0x00007f85f15a6710 @name="Tokyo", @utc_offset=nil, @tzinfo=#<TZInfo::TimezoneProxy: Asia/Tokyo>>

Time.current
# => Mon, 29 Jul 2019 15:59:00 JST +09:00

BroadcastMessage
# => BroadcastMessage(id: integer, message: text, starts_at: datetime_with_timezone, ends_at: datetime_with_timezone, created_at: datetime_with_timezone, updated_at: datetime_with_timezone, color: string, font: string, message_html: text, cached_markdown_version: integer)

BroadcastMessage.first.starts_at
# => 2018-07-24 10:00:00 UTC

BroadcastMessage.first.starts_at.class
# => Time

Related issues

Nothing

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