Handle symbol header

[brissmyr]

Clearance sends a symbol header (:clearance) instead of string, which crashes the parsing.

[2] pry(#<Castle::HeaderFormatter>)>

From: /Users/brissmyr/.rvm/gems/ruby-2.2.4/gems/castle-rb-3.0.0/lib/castle/header_formatter.rb @ line 7 Castle::HeaderFormatter#call:

    5: def call(header)
    6:   binding.pry
 => 7:   header.gsub(/^HTTP(?:_|-)/i, '').split(/_|-/).map(&:capitalize).join('-')
    8: end

[1] pry(#<Castle::HeaderFormatter>)> header
=> "action_dispatch.request.content_type"
[2] pry(#<Castle::HeaderFormatter>)>

From: /Users/brissmyr/.rvm/gems/ruby-2.2.4/gems/castle-rb-3.0.0/lib/castle/header_formatter.rb @ line 7 Castle::HeaderFormatter#call:

    5: def call(header)
    6:   binding.pry
 => 7:   header.gsub(/^HTTP(?:_|-)/i, '').split(/_|-/).map(&:capitalize).join('-')
    8: end

[1] pry(#<Castle::HeaderFormatter>)> header
=> :clearance
[2] pry(#<Castle::HeaderFormatter>)>
Completed 500 Internal Server Error in 179877ms

NoMethodError (undefined method `gsub' for :clearance:Symbol):
  app/controllers/sessions_controller.rb:11:in `create'