Instead of using onSendingHeaders, we only invoke a custom handler if a body has not been written to.
Remove Nancy dependency.
Just verifying that the tests work with HttpListener (going to remove after this commit)
Revert "Just verifying that the tests work with HttpListener (going to remove after this commit)"

This reverts commit 88e9b06e9a853d68d6af0951f59f7e0a2607ea33.