[Bitbucket]: PR: Edit fails

**Describe the bug**
When editing a PR summary, the update fails with the following error

EDIT: This also happens when editing the title of the PR

**To Reproduce**
Steps to reproduce the behavior:
1. Open an existing PR
2. Edit the summary
3. Click `Save`
4. See error


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**A side note: the api call being used here is using `BASIC` auth. Can this be upgraded to use Bearer token?**

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**Expected behavior**

I should be able to edit the summary successfully.

**Screenshots**
If applicable, add screenshots to help explain your problem.

**Environment and Version (please complete the following information):**
- Atlassian Plugin Version (3.4.6)
- Visual Studio Code Version (x.x.x)
- Is this a local or cloud environment? (Cloud | Local)
- If Cloud, Browser (chrome, safari)
- If Local, OS (macOS, windows, linux)

**Additional context**
Add any other context about the problem here.

Start with the Bitbucket pull-request edit flow described in issue #128 and reproduce the failure by editing the summary or title and selecting Save. Check the request used for that update, including the reported BASIC authentication, and verify that both fields can be saved successfully without the error.

Written by the indexing model from the issue text.