Standardize Response Headers from all Adapters

Now, all adapters returns Array when response have more than on header. Used
"Set-Cookie" for example on tests. This commit, fix issue #69 and closes pull
request #52.
preparing v2.0.1
refactored request#set_cookies to accept an array
preparing v2.0.2
merge branch 'set-cookies'

Conflicts:
	CHANGELOG.md
	lib/httpi/adapter/net_http.rb