emacs: notmuch-show-view-raw-message clears buffer, makes it read-only

emacs: notmuch-show-view-raw-message clears buffer, makes it read-only

notmuch-show-view-raw-message() re-uses buffer created with same
name (same Message-Id:) but it did not erase it before filling.
If this ever happened, there were duplicated (potentially overlapping)
content in the buffer. Now this is fixed.
Apparently since emacs 24.5 the (view-buffer) makes the buffer read-only;
so this problem would not have happened there, just that
notmuch-show-view-raw-message() failed. This is fixed by setting
inhibit-read-only t before erasing and filling the buffer. The emacs 24.5
feature having raw message buffer read-only is also now explicitly set to
the buffer so the same experience is available with emaces < 24.5.