Add a test that exposes a data race in (*Martini).createContext()
Add a test that exposes a data race in (*Martini).createContext()

Run `go test -race` to reproduce. (*Martini).handlers is modified simultaneously by several goroutines.

cjhubert/martini,mingderwang/martini,beni55/martini,mirrr/martini,mtojek/martini,SmarkSeven/martini,coffeehc/martini,anilanar/martini,jjz/martini,mikeqian/martini,jeff235255/martini,cocotyty/martini,franciscocpg/martini,zj8487/martini,huamichaelchen/martini,wxdublin/martini,OscaRoa/martini,trigrass2/martini,williamzhang2013/martini,imjerrybao/martini,aperera/martini,w4ngyi/martini,soheilbm/martini,folpindo/martini,ansiz/martini,b54898533/martini,lcwxz1989/martini,basakakoel/martini,wddqing/martini,alskjstl/martini,CestusMagnus/martini,kingland/martini,gcnonato/martini,kalashnikov/martini,sandyway/martini,treetopllc/martini,go-martini/martini,Berkaroad/martini,sygool/martini,Affix/martini