Add headers rendering and example of basicauth

[zhouzhuojie]

Previously the headers in reply_http and send_http are not rendered with the context, this PR adds that so that headers can also be dynamic.

Example:

- key: base64-basicauth-with-env
  kind: Behavior
  expect:
    http:
      method: POST
      path: /base64-basicauth-with-env
  actions:
    - reply_http:
        headers:
          Authorization: '{{ printf "foobar:%s" (env "BASIC_AUTH_PASS") | b64enc }}'
        status_code: 200

[michelleheh]

Should we add the same logrus to be consistent with the render of body failure?

[michelleheh]

Never mind, I see that it's happening in renderHeaders.

[michelleheh]

Works great! 👍Will you consider adding a unit test? Also, it will be great to add some information to the README.

[michelleheh]

Never mind, I see that it's happening in renderHeaders.