mu4e: improve mu4e-fill-paragraph

Improve mu4e-fill-paragraph for some corner-cases where there is
trailing whitespace. Patch by holtzermann17.

mu4e/mu4e-compose.el

@@ -355,7 +355,9 @@ simply executes `fill-paragraph'."
   (if mu4e-compose-format-flowed
     (let ((fill-column (point-max))
        (use-hard-newlines nil)); rfill "across" hard newlines
+      (delete-trailing-whitespace)
       (fill-paragraph nil region))
+    (delete-trailing-whitespace)
     (fill-paragraph nil region)))
 
 (defun mu4e-toggle-use-hard-newlines ()