Allow Set-Cookies for all responses

[smancke]

With this PR, it is also possible that other responses set a cookie.

The PR is based on #31, so should be merged afterwards,

[coveralls]

Coverage increased (+0.3%) to 92.792% when pulling c2e32ef on allow-set-cookies-2 into d4e78f0 on master.

[deberth]

Looks good to me.
Is the header field set-cookie always written case-sensitive as "Set-Cookie"? So we don't have to do something like "toLower()" in the "contains" method?

[smancke]

@deberth
Yes, the Golang http api unified the headers:
// HTTP defines that header names are case-insensitive. The
// request parser implements this by using CanonicalHeaderKey,
// making the first character and any characters following a
// hyphen uppercase and the rest lowercase.