[REST] fix headersonly flag for BINARY responses

oops. was to quick.
Now it seams to be okay:

    curl -v http://localhost:18332/rest/tx/3cfe5051f7efe2702c5720b1ce6f5984181b9172a399935ff22a3e08df143c91/dat > test.bin

-

    < Content-Length: 109

-
    ls -la test.bin
    -rw-r--r--  1 jonasschnelli  staff  109 20 Nov 14:13 test.bin