Fix peek--get-references for lsp-use-plists (#743)

When `lsp-use-plists` is t, `lsp-request` returns a single plist
(of form (:key val ...) or a list of plists.

Previously, when `lsp-use-plists` was t and a single plist was returned,
the plist wasn't wrapped into a list, causing the subsequent code to
fail.

Author: erikarvstedt

lsp-ui-peek.el

@@ -726,7 +726,10 @@ references.  The function returns a list of `ls-xref-item'."
 Returns item(s)."
   (-when-let* ((locs (lsp-request method params))
                (locs (if (listp locs)
-                         locs
+                         (if (symbolp (car locs))
+                             ;; A single plist was returned
+                             (list locs)
+                           locs)
                        (if (vectorp locs)
                            (append locs nil)
                          (list locs)))))