Update response.py

Just something I'm monkey patching locally to allow setting the same header multiple times.

Useful for things like link headers, and setting cookies.
Update response.py
Update test_headers.py
Update test_headers.py
Update response.py
feat(Response): Add a response helper for setting the Link header

This patch implements an "add_link" method for the Response class that
apps can use to add one or more Link header values to a response.

See also: https://tools.ietf.org/html/rfc5988

Closes #291
Fix numbered list.
Merge pull request #305 from rubik/patch-2

Fix numbered list.
Merge pull request #295 from kgriffs/add_link

feat(Response): Add a response helper for setting the Link header
Merge pull request #273 from xiaclo/patch-1

Allow duplicate headers to be appended
test(Response): Improve resp.append_header tests

This patch improves test coverage of resp.append_header and makes the
approach more consistent with the other functional tests in the same
module.
Bump version to 0.2.0a1
fix(Request): SCRIPT_NAME may not always be present in the WSGI env