Poor description of the body encoders, too much details is hidden.
Add moar info about multipart attachments and implementation logic (in the text: "if you pass a Buffer ..., if you pass a file description object"; Explain the file description object format, pls).
Add link to multipart/form-data spec if you refers to it in text.
How I can to implement custom encoder? What about inheritance?
What is about errors?
When body processing will done? In ctor? or duering the #execute() running?
fixed #70