Fix POST bug for several providers

Due to the specifial handling for syntax, some providers would have an
extra POST parameter, which then caused some providers to reject the
paste upload completely.

This change makes it so that if a provider doesn't specify special
language rules, don't try to add a syntax POST parameter.

webpaste.el

@@ -152,13 +152,9 @@ precalculated, and also available both for pre and post request access.")
                  (let ((provider-lang-alist (cdr (assoc provider-uri webpaste/provider-lang-alists))))
                    ;; Fetch language name for this major mode for this provider
                    (let ((language-name (cdr (assoc major-mode provider-lang-alist))))
-                     ;; If not set correctly, get the fundamental-mode one which should be plaintext
-                     (unless language-name
-                       (setq language-name (cdr (assoc 'fundamental-mode provider-lang-alist))))
-
-                     ;; Append language to the post-data
-                     (cl-pushnew (cons post-lang-field-name language-name) post-data)))
-
+                     (if (and post-lang-field-name language-name)
+                       ;; Append language to the post-data
+                       (cl-pushnew (cons post-lang-field-name language-name) post-data))))
                  post-data))
   "Predefined lambda for building post fields.")