fix not to touch content_type when posting a buffer

“is(data).a(Object)” returns true when “data” is a Buffer. And
previous code is resetting type as “form” if the type isn't set to
“json”.

This commit patches to check a Buffer specifically using
“Buffer.isBuffer” method.
add newline at end of file
Merge pull request #18 from hatemogi/buffer_with_type

fix not to touch content-type when posting a buffer