spamassassin: fixes Haraka crash when spamd_response.headers not defined

happens when local SA config doesn't enable that header and results in this:

[CRIT] [-] [core] ReferenceError: Invalid left-hand side in assignment
    at Socket.<anonymous> (/usr/local/haraka/plugins/spamassassin.js:88:13)

tested again Haraka 2.5.0(ish) and SpamAssassin 3.4.0