Suggestion for supporting single header fields.

Motivation:
At the moment if you want to return a HTTP header containing multiple
values you have to set/add that header once with the values wanted. If
you used set/add with an array/iterable multiple HTTP header fields will
be returned in the response.

Note, that this is indeed a suggestion and additional work and tests
should be added. This is mainly to bring up a discussion.

Modifications:
Added a flag to specify that when multiple values exist for a single
HTTP header then add them as a comma separated string.

Result:
Allows for responses to be smaller.
I think it is tough to come up with a good name here because we are wrapping `add` and `set`.  I'm wondering if we can come up with a name that is less biased toward `add` or `set`.  Does `HeaderComposer` make sense, or spark any new thoughts?