Restore support for cookies when using custom headers

[nielsbuus]

This commit configures cookies AFTER configuring headers. This is necessary because the header configuration method resets existing headers, including cookies.

[kronn]

How can me move this one forward? The broken build seems to be unrelated to the change. I would very much like to switch from the PRs fork to an official gem version with that change.

[tjarratt]

@nielsbuus I appreciate the contributions and all but ... did you really mean to include the changes to README.md along with the code change?

[nielsbuus]

@tjarratt No, I didn't. If you look at the history, there are two weeks between the commits. The first commit is the fix and the second commit was added to the repository when I grew tired of waiting and had to document the motivation for the fork.

Since the pull request merges our master into savonrb master, the update to our master inadvertently became part of this open pull request.