Allow uploading directories

[Ambrevar]

At the moment sending a request with a parameter like

("file" #p"/path/to/dir/" :content-type "application/x-directory")

fails because send-content tries to read the dir.
Suggested fix in send-content:

((pathnamep content)
           (when (uiop:file-exists-p content)
             (with-open-file (from content :element-type 'octet)
               ;; calls itself with a stream now
               (send-content from stream))))

For context, I'm trying to upload directories to IPFS:

http://docs.ipfs.tech.ipns.localhost:8080/reference/kubo/rpc/#api-v0-add

[Ambrevar]

Also then the filename must be set properly in make-form-data-function.

Suggested fix:

(defun basename (pathname)
  (first (last (pathname-directory
                    ;; Ensure directory _after_ truenamizing, otherwise if
                    ;; non-directory file exists it may not yield a directory.
                    (uiop:ensure-directory-pathname
                     (uiop:ensure-pathname pathname :truenamize t)))))

(defun make-form-data-function (parameters boundary external-format-out)
  ; ...
  (pathname (basename file-source))
;...

[Ambrevar]

@stassats Friendly ping :)

[stassats]

I don't see a pull request linked to this.

[Ambrevar]

See #127.