I've found the issue which seems to be almost the same, but it's closed and I cannot find a way to change a target branch to merge pull request in.
1) Create a public repository "A" with branches master, features
2) Create a new user, fork the "A" with a new public one (let it be "B")
3) Make some changes in the "B"
4) Create a pull request to the "A", select branch master
5) Log in with the administrator of the "A"
6) Check your "Pull requests" - there is no chance to edit the target branch (it's set to "master" as the user selected). If you press "Merge", all the changes will be merged into master.
At the same time, the user who creates pull request do have a chance to specify a branch to merge request into.
It would be perfect if there is a chance to merge pull requests into custom branches, may be it should be created during pull request acceptance. It seems to be very useful in case of code review with fisheye+crucible, for example - you can merge the request into a branch, create a review in your crucible system, make some testing and then merge into master if needed.
Thanks in advance!