spamassassin: fixed 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)
Merge pull request #583 from msimerson/spamassassin

spamassassin: fixes Haraka crash when spamd_response.headers not defined