Configurable text_based?(content_type)

[n-epifanov]

Hi,

I've run into a problem of PDF dumping into log file. What do you think about making it configurable like:

HttpLog.configure do |config|
  config.log_response_filter    = proc do |content_type|
   content_type =~ /^text/ ||
        content_type =~ /^application/ && content_type != 'application/octet-stream'
    end

I'd make a PR.

[n-epifanov]

Thanks!