Fail to change default branch if branch name have uppercase chatacter

[ncwgf]

• Gitea version (or commit ref): 1.1.1+6-gbbe6aa34 ~ 1.1.2
• Database (use [x]):
  • PostgreSQL
  • MySQL
  • MSSQL
  • SQLite
• Can you reproduce the bug at https://try.gitea.io:
  • Yes (provide example URL)
  • No
  • Not relevant
• Log gist:

Description

When changing default branch at repo setting, branch page
select branch name which have capital character and click Update Setting.
Return screen show success message but no effect was taken.
If select lowercase branch, it success and work as expect

Possible related code: https://github.com/go-gitea/gitea/blob/master/routers/repo/setting.go#L483