Whitelist the headers reflected in requests.
Whitelist the headers reflected in requests.

I originally had it copy all the headers from the request, but that ends
up getting User-Agent and the X-AppEngine-Country that App Engine
apparently adds.

Here is a request received at the bridge when all headers are copied:

POST / HTTP/1.1
Content-Length: 272
Accept-Encoding: identity
User-Agent: Go 1.1 package http AppEngine-Google; (+http://code.google.com/appengine)
Host: 127.0.0.1:31337
X-Session-Id: udSArUKd17vnCm4BoN/rTe7yHKGm4co/O6WmHpCat9c=
Content-Type: application/octet-stream
X-Appengine-Country: ZZ

Here is a request with X-Session-Id and Content-Type whitelisted:

POST / HTTP/1.1
Content-Length: 273
Accept-Encoding: gzip
User-Agent: AppEngine-Google; (+http://code.google.com/appengine)
Host: 127.0.0.1:31337
X-Session-Id: 6gqbdmC2shb2sjWwxI0BzuVARchZirn+4pLw/xNbfsw=
Content-Type: application/octet-stream

scucoco/meek,projectarkc/meek,projectarkc/meek,wyuelin/mymeek,infinity0/meek,infinity0/meek,scucoco/meek,projectarkc/meek,arlolra/meek,gorshell/forkmeek,Yawning/meek,arlolra/meek,gorshell/forkmeek,infinity0/meek,Yawning/meek,wyuelin/mymeek,infinity0/meek,wyuelin/mymeek,projectarkc/meek,gorshell/forkmeek,scucoco/meek,projectarkc/meek,infinity0/meek,gorshell/forkmeek,wyuelin/mymeek,Yawning/meek,arlolra/meek,Yawning/meek