Log headers with toString() instead of own impl

Header has a proper toString() method implementation that provides
proper functionality when System.out.print-ing the contents of
the header: instead of concatenating the string values, which can
be null, resulting in possibly "null: null" as a header option,
the toString() method correctly replaces a null occurrence with
the empty string.

Case in point, when logging the response you'll get:

    <--- HTTP 200 http://api.example.com/ (74ms)
    null: HTTP/1.1 200 OK

Which would look much better as:

    <--- HTTP 200 http://localhost:8081/somtoday/rest/ (74ms)
    HTTP/1.1 200 OK
Merge pull request #327 from dashorst/patch-1

Log headers with toString() instead of own impl