Strip properties from mu4e-completing-read-function results.

Some completion engines (like "flx") decorate the strings that they
return.  If MU4E passes such a string down to MU, the "format" call
preserves the text properties in the generated S-expression, producing
an invalid query.  MU4E itself has no interest in those decorations,
so strip them out as early as possible from all prompts that use
mu4e-completing-read-function.

mu4e/mu4e-contrib.el

@@ -198,8 +198,9 @@ buffers found, compose a new message and then attach the file."
                 (if (= (length bufs) 1)
                     (get-buffer (car bufs))
                   (let ((prompt (mu4e-format "%s" "Attach to buffer")))
-                    (funcall mu4e-completing-read-function prompt
-                             bufs))))
+                    (substring-no-properties
+                     (funcall mu4e-completing-read-function prompt
+                              bufs)))))
         ;; setup a new mail composition buffer
         (if (y-or-n-p "Compose new mail and attach this file? ")
             (progn (mu4e-compose-new)