Ensure that uid in OAuth is a string

In our company we use Redmine as OAuth provider (with https://github.com/suer/redmine_oauth_provider and https://github.com/suer/omniauth-redmine on GitLab side). In this case `uid` returned from Redmine is its database ID as integer. This leads to:

	PG::Error: ERROR:  operator does not exist: character varying = integer
	LINE 1: ..."."provider" = 'redmine' AND "users"."extern_uid" = 422  ORD...
	                                                             ^
	HINT:  No operator matches the given name and argument type(s). You might need to add explicit type casts.
	: SELECT  "users".* FROM "users"  WHERE "users"."provider" = 'redmine' AND "users"."extern_uid" = 422  ORDER BY "users"."id" DESC LIMIT 1
	Completed 500 Internal Server Error in 27ms
	 
	ActiveRecord::StatementInvalid (PG::Error: ERROR:  operator does not exist: character varying = integer
	LINE 1: ..."."provider" = 'redmine' AND "users"."extern_uid" = 422  ORD...
	                                                             ^
	HINT:  No operator matches the given name and argument type(s). You might need to add explicit type casts.
	: SELECT  "users".* FROM "users"  WHERE "users"."provider" = 'redmine' AND "users"."extern_uid" = 422  ORDER BY "users"."id" DESC LIMIT 1):
	  lib/gitlab/oauth/user.rb:56:in `find_by_uid_and_provider'
	  lib/gitlab/oauth/user.rb:14:in `find'
	  app/controllers/omniauth_callbacks_controller.rb:36:in `handle_omniauth'
	  app/controllers/omniauth_callbacks_controller.rb:4:in `block (2 levels) in <class:OmniauthCallbacksController>'
	  app/controllers/application_controller.rb:59:in `set_current_user_for_thread'

Casting `uid` to string fixes this issue.