Ideally, the HttpClient/HttpServer adapters should maintain the invariant that
the behavior of a given client is the same as the behavior of
newHttpClient(newHttpService(client)).  Prior to this change, the HttpClient
wrapper lazily called request(), so a client whose request() eagerly threw an
exception could produce a different exception when called directly versus when
wrapped -- the laziness allowed additional code to run.  This was particularly
evident when making a request with a body, since code using a wrapped client
would be able to set up a subsequent BlockedWrite, eventually resulting in a
"read end of pipe was aborted" exception instead of the actual exception.

This change makes HttpClientAdapter::request() execute the wrapped request()
eagerly.

(Note that it partially undoes 90d48343... Hopefully, it preserves the desired
behavior added there.)