Support single 'Set-Cookie' header via 'setCookie' key in result obj

[riotrah]

• Implements #47
• Enables user to define a single Set-Cookie header in their response object via a setCookie key, which will be defined on headers as Set-Cookie.
• Empty setCookie will not add to headers

Only supports a single 'Set-Cookie' as multiValueHeaders is not yet supported in this release.

If we can get multiValueHeaders in I'll implement multiple Set-Cookies.

Also, the assertions I added may not be in the most useful of places, currently we run the setCookie assertion 4 times in the .processResult() tests.

[richardhyatt]

Merged changes by hand