Reduce redundant code in "div-open"

This approach includes the necessary conditionals in the
bindings of the parameters themselves, avoiding to have
to separate but nearly identical structures in the branches of the
if clause.

Additionally it fixes an issue when both class and id are present
(where the space was missing in between). Now an arbitrary number of tags can be joined in an opening tag

annotate/export-html.ily

@@ -36,33 +36,24 @@
    (format "class=\"~a\"" class))
 #(define (idify-html-tag id)
    (format "id=\"~a\"" id))
-#(define (delimit-html-tags div-type tags)
-   (format "<~a ~a>" div-type tags))
+#(define (delimit-html-tags tags)
+   (format "<~a>" (string-join tags " ")))
 
 % open div with unique tags
 #(define (div-open type ann-or-string nest-level)
    ;; if class = string, don't check for an id. otherwise it
    ;; is an ann props list, so check for an id and apply if necessary
-   (let ((trailing-line (if (= nest-level 0) "\n" "")))
-     (if (string? ann-or-string)
-         (let* ((class (classify-html-tag ann-or-string))
-                (div-type (symbol->string (getChildOption
-                                           `(scholarly annotate export html divs)
-                                           type)))
-                (div-tag (delimit-html-tags div-type class))
-                (div-begin (format "~a~a" (nest-indent div-tag nest-level) trailing-line)))
-           (append-to-output-stringlist div-begin))
-         (let* ((ann ann-or-string)
-                (class (classify-html-tag "annotation"))
-                (div-type (symbol->string (getChildOption
-                                           `(scholarly annotate export html divs)
-                                           type)))
-                (id (if (assq-ref ann 'html-id)
-                        (idify-html-tag (assoc-ref ann 'html-id))
-                        ""))
-                (div-tags (delimit-html-tags div-type (string-append class id)))
-                (div-begin (format "~a~a" (nest-indent div-tags nest-level) trailing-line)))
-           (append-to-output-stringlist div-begin)))))
+   (let* ((trailing-line (if (= nest-level 0) "\n" ""))
+          (div-type (symbol->string
+                     (getChildOption `(scholarly annotate export html divs) type)))
+          (class (classify-html-tag (if (string? ann-or-string) ann-or-string "annotation")))
+          (id
+           (let ((html-id (and (not (string? ann-or-string))
+                               (assq-ref ann-or-string 'html-id))))
+             (if html-id (idify-html-tag html-id) "")))
+          (div-tags (delimit-html-tags (list div-type class id)))
+          (div-begin (format "~a~a" (nest-indent div-tags nest-level) trailing-line)))
+     (append-to-output-stringlist div-begin)))
 
 % close any div
 #(define (div-close type nest-level)