Reinstate yield() after @async writebody(...).

Was removed in b602c4e :

> Remove yeild() after @async writebody. This should increase the chance
> that the startread() realises that the connection is dead before the
> body is written.

This change appears to have caused problems in other situations:
JuliaCloud/AWSS3.jl#26 (comment)

The removal was driven by nanosoldier/GitHub.jl issues with POST
requests on long-held connections.
#220 (comment)

GitHub.jl now handles this with the `idle_timeout=` option.
JuliaWeb/GitHub.jl#109

src/StreamRequest.jl

@@ -54,6 +54,7 @@ function request(::Type{StreamLayer}, io::IO, request::Request, body;
 
             if iofunction == nothing
                 @async writebody(http, request, body)
+                yield()
                 startread(http)
                 readbody(http, response, response_stream)
             else