class Rack::ContentType

default.
When no content type argument is provided, “text/html” is the
use Rack::ContentType, “text/plain”
Builder Usage:
Sets the content-type header on responses which don’t have one.

def call(env) 

def call(env)
  status, headers, _ = response = @app.call(env)
  unless STATUS_WITH_NO_ENTITY_BODY.key?(status.to_i)
    headers[CONTENT_TYPE] ||= @content_type
  end
  response
end

def initialize(app, content_type = "text/html") 

def initialize(app, content_type = "text/html")
  @app = app
  @content_type = content_type
end