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.
Extract this `10` to a `static final` in `DefaultTextHeaders` at the top of the file so we can make it more visible and add some comments.  In the future this may make it more feasible to be controllable via system property (if needed).