Fixes #1932: Ignores case for pull request autolink in Bitbucket Server

[smartinio]

Description

Fixes #1932
Makes pull request autolink case insensitive, so it also works with Bitbucket Server 7
Now matches both "pull request #" and "Pull request #"

Checklist

• I have followed the guidelines in the Contributing document
• My changes follow the coding style of this project
• My changes build without any errors or warnings
• My changes have been formatted and linted
• My changes include any required corresponding changes to the documentation
• My changes have been rebased and squashed to the minimal number (typically 1) of relevant commits
• My changes have a descriptive commit message with a short title, including a Fixes $XXX - or Closes #XXX - prefix to auto-close the issue that your PR addresses

[eamodio]

Thank you for your contribution!