aws/service/rds: Don't auto-sign (preSignUrl) for same region operations

[omeid]

Hello!

We are using an automation tool that creates read replicas of an instance in different regions, however, it results into an error if we try to create a read replica in the same region. precisely, the sdk causes an error if you indicate a source region for a replica when the source and replica is in the same region.
This commit fixes this issue.

Related issues are #1127 and #1128.

AWS does not like preSignedUrl when it is not required and returns a 400
error:

Error creating DB Instance: InvalidParameterCombination: Your request
does not require the preSignedUrl parameter. Please remove the
preSignedUrl parameter and try your request again.

This commit adds the check to escape auto-sign when a request source
and destination is the same region is the same.

[omeid]

@jasdel What needs to be done to get this merged? This is blocking creating read replicas in the same region with terraform :(

[jasdel]

Thanks for taking the time to investigate and create this PR @omeid. The change looks good.

I couldn't find explicit documentation that states that a call for a resource within the same region must not include a presignedURL. I'll forward this to the service's team to help improve the documentation of this feature.

[jasdel]

The change looks good, the only request I have would be to update the service/rds/customizations_test.go file to include cases for same region.

Also, if you'd prefer we can add the tests before merging this change in.

[omeid]

Thanks for the follow up.

Unfort I am not immediately familiar with the test setup and it may take a while for me to fix it up, so I am keen to take your suggestion of adding tests before merging this in.

Cheers

[jasdel]

Created #2631 with unit tests for this bug fix.