HttpResponse::getBodyAsJson is declared by has no definition

http::HttpReponse::getBodyAsJson is declared in the header but there is no definition of it. Thus if one attempts to use it, one is rewarded with an unresolved symbol:

/home/tmark/labs/build/keadev/open/git.1730/kea/src/lib/config/tests/cmd_http_listener_unittests.cc:384: undefined reference to `isc::http::HttpResponse::getBodyAsJson() const'
collect2: error: ld returned 1 exit status