511: GrizzlyAsyncHttpProvider not honoring usePreemtiveAuth

consisting of a modified
providers/grizzly/src/main/java/org/asynchttpclient/providers/grizzly/filters/AsyncHttpClientFilter.java

to make the preemptive auth mode working for grizzly.

a modified
api/src/test/java/org/asynchttpclient/async/BasicAuthTest.java

to correctly verify whether the auth header is actually sent in the initial message.

regards, aki