Merging a pull-request without specifying a body overwrites close_source_branch value

When merging a pull-request, via the Bitbucket REST API, if no body is provided then the original close_source_branch value (on the PR) is both ignored, and overwritten.

According to the API documentation:

close_source_branch boolean
Whether the source branch should be deleted. If this is not provided, we fallback to the value used when the pull request was created, which defaults to False*

https://developer.atlassian.com/bitbucket/api/2/reference/resource/repositories/%7Busername%7D/%7Brepo_slug%7D/pullrequests/%7Bpull_request_id%7D/merge

Steps to replicate

• Create a pull-request, and specify close_source_branch

POST https://api.bitbucket.org/2.0/repositories/<owner>/<slug>/pullrequests
{   ...
    "close_source_branch": true
}

• Retrieve the pull request - confirm that close_source_branch is still true

GET https://api.bitbucket.org/2.0/repositories/<owner>/<slug>/pullrequests/<id>

• Merge the PR, without a request body

POST https://api.bitbucket.org/2.0/repositories/<owner>/<slug>/pullrequests/<id>/merge

• Retrieve the pull request

GET https://api.bitbucket.org/2.0/repositories/<owner>/<slug>/pullrequests/<id>

Expected: close_source_branch is still true, and the source branch was closed after the merge was completed

Actual: close_source_branch is overwritten to be false, and the source branch remains open after the merge was completed