In UsersController#calendar_activities, when Date isn't parsable, fallback to Date.today
In UsersController#calendar_activities, when Date isn't parsable, fallback to Date.today

For some reason, GoogleBot accesses /u/:username/calendar_activities without
a :date param, but then the view was trying to call #to_s(:short) which
doesn't exist on nil, leading to the following Sentry report:
https://sentry.gitlap.com/gitlab/gitlabcom/issues/1182/

SVArago/gitlabhq,martijnvermaat/gitlabhq,iiet/iiet-git,Soullivaneuh/gitlabhq,jirutka/gitlabhq,Soullivaneuh/gitlabhq,jirutka/gitlabhq,daiyu/gitlab-zh,larryli/gitlabhq,larryli/gitlabhq,screenpages/gitlabhq,daiyu/gitlab-zh,openwide-java/gitlabhq,screenpages/gitlabhq,axilleas/gitlabhq,dplarson/gitlabhq,stoplightio/gitlabhq,icedwater/gitlabhq,htve/GitlabForChinese,stoplightio/gitlabhq,shinexiao/gitlabhq,LUMC/gitlabhq,t-zuehlsdorff/gitlabhq,shinexiao/gitlabhq,darkrasid/gitlabhq,iiet/iiet-git,martijnvermaat/gitlabhq,dwrensha/gitlabhq,SVArago/gitlabhq,LUMC/gitlabhq,iiet/iiet-git,t-zuehlsdorff/gitlabhq,mmkassem/gitlabhq,mr-dxdy/gitlabhq,mmkassem/gitlabhq,shinexiao/gitlabhq,icedwater/gitlabhq,screenpages/gitlabhq,allysonbarros/gitlabhq,dreampet/gitlab,ttasanen/gitlabhq,ttasanen/gitlabhq,axilleas/gitlabhq,t-zuehlsdorff/gitlabhq,martijnvermaat/gitlabhq,LUMC/gitlabhq,dplarson/gitlabhq,ttasanen/gitlabhq,t-zuehlsdorff/gitlabhq,daiyu/gitlab-zh,dwrensha/gitlabhq,stoplightio/gitlabhq,martijnvermaat/gitlabhq,jirutka/gitlabhq,stoplightio/gitlabhq,htve/GitlabForChinese,mmkassem/gitlabhq,iiet/iiet-git,mmkassem/gitlabhq,shinexiao/gitlabhq,icedwater/gitlabhq,ttasanen/gitlabhq,SVArago/gitlabhq,SVArago/gitlabhq,larryli/gitlabhq,openwide-java/gitlabhq,icedwater/gitlabhq,htve/GitlabForChinese,LUMC/gitlabhq,axilleas/gitlabhq,larryli/gitlabhq,darkrasid/gitlabhq,dreampet/gitlab,screenpages/gitlabhq,mr-dxdy/gitlabhq,openwide-java/gitlabhq,mr-dxdy/gitlabhq,Soullivaneuh/gitlabhq,htve/GitlabForChinese,dwrensha/gitlabhq,axilleas/gitlabhq,mr-dxdy/gitlabhq,dplarson/gitlabhq,jirutka/gitlabhq,allysonbarros/gitlabhq,dwrensha/gitlabhq,dplarson/gitlabhq,Soullivaneuh/gitlabhq,darkrasid/gitlabhq,dreampet/gitlab,allysonbarros/gitlabhq,allysonbarros/gitlabhq,openwide-java/gitlabhq,daiyu/gitlab-zh,darkrasid/gitlabhq,dreampet/gitlab