This method is deprecated or moved on the latest stable version. The last existing version (v5.1.7) is shown here.
build_hsts_header(hsts)
http://tools.ietf.org/html/rfc6797#section-6.1
# File actionpack/lib/action_dispatch/middleware/ssl.rb, line 98 def build_hsts_header(hsts) value = "max-age=#{hsts[:expires].to_i}" value << "; includeSubDomains" if hsts[:subdomains] value << "; preload" if hsts[:preload] value end