fix type coercion bug when setting Cache-Control header maxAge to zero.

HTTP Cache-Control header cache-directive max-age is allowed to be zero per http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.9.3
fix type coercion bug when setting Cache-Control header maxAge to zero.

HTTP Cache-Control header cache-directive max-age is allowed to be zero per http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.9.3
Merge pull request #403 from ccannell/bug/maxAge-set-to-zero

fix type coercion bug when setting Cache-Control header maxAge to zero.