wip
workaround like a baws for Android 4.0.3 ssl engine bug:
https://code.google.com/p/android/issues/detail?can=2&start=0&num=100&q=&colspec=ID%20Type%20Status%20Owner%20Summary%20Stars&groupby=&sort=&id=9307
continuation bug fix if onCOmpleted is called twice. should probably assert on that...
fix http client bug if error occurs before header is received. callback is never invoked.