REST API can return unparseable XML output (BB-4508)

Attached file is a result of
curl --user 'kbrazulewicz:XXXXXXXXXXXX' https://api.bitbucket.org/1.0/repositories/atlassian/bamboo/branches/?format=xml

The reason is a bizarre branch name "BSP-6780 - support for mercurial subrepos with relative path" which shouldn't be used as an XML tag name.