Somebody havent heard about SemVer yet. The Ctx.GetReqHeaders() return value
slightly changed (string to []string), which now requires to first join the slice
before checking whether it contains the given string

* add dependency to allow limiting possible values, though actual
  validation doesnt happen yet
* make variable that indicates health local instead of global

The three dots ('./...') are required to ensure recursive dependency
installation, meaning dependencies of sub-packages.