Add PSGI Streaming - Updated

Refactored so that when $body is a subref, we just return it.

Updated the tests and docs to reflect this. 

I've still a minor question around the headers:

       #my $writer = $respond->([200, ['Content-Type' => 'text/plain']]);   # this works fine
       #my $writer = $respond->([ $self->query->psgi_header ]);             # this doesn't work?
       my $writer = $respond->([ $self->_send_psgi_headers ]);              # this works, but uses an internal call - perhaps it should be made public?