Added get getContentVersionBody and getAttachmentBody

[danielpetrica]

Based on code provided in issue https://github.com/omniphx/forrest/issues/179 by @ShintaroNitta and @zack-carlson

[danielpetrica]

Drafted as I need to integrate facade and the readme file

[danielpetrica]

I tested it with the client credential workflow only

[danielpetrica]

Hi I'm using the eloquent package and this library in a site we're developing at work. I found out that we need this code to be able to download files uploaded to sales force with the contentVersion flow. I managed to upload the file but I incurred the same error as in ticket https://github.com/omniphx/forrest/issues/179. Instead of overwriting the class, I think it is more elegant to extend the base class so we can use the method in all authentication classes.

[omniphx]

Hi @danielpetrica, thanks for the contribution. I'll go and merge this in since it shouldn't be a breaking change for the Web Auth flow.

[danielpetrica]

Thank you for the merge and for the packet. It's making my job eaier ❤️