[#5] Emit multiple header values

... instead of concatenating. Per RFC2616, any header that allows
multiple values may be represented EITHER by concatenating values with a
comma OR by emitting the header multiple times (though order MUST be
maintained). In the case of the Set-Cookie header, you CAN NOT
concatenate cookie declarations in a single header value. As such, this
patch ensures that all header declarations will always work.
[#5] Wrote tests verifying header emit changes

Two tests:
- One verifying multiple header lines are emitted for the same header if
  multiple values are present.
- One verifying that those header lines are emitted in the same order
  added.
Merge branch 'hotfix/5'

Close #5