Add receivedHTTPHeaders array to public properties

This is my first-ever pull request; apologies if I'm not doing this right. 

For my application I need to get the actual headers served up by the web service in the event of a failure. However the receivedHTTPHeaders property is private in the current release. My change puts that property in the public so my app can access them. I'm not sure if there's a Very Good Reason™ why the headers aren't currently available, but this change was useful to me.

Thanks for reading! This has been such a terrific library of code to use, so thanks for all your great work.