Remove connection header from grpcwebproxy

[crlssn]

This PR fixes the issue expressed in #568. Basically, when you are sending a gRPC request from a web client through the proxy, multiple headers are added of which connection is denying the actual connection to be established.

Error message with the connection header present:

INFO[0000] listening for http on: [::]:8080             
INFO[0000] pickfirstBalancer: HandleSubConnStateChange: 0xc00015e200, READY  system=system
INFO[0075] transport: loopyWriter.run returning. connection error: desc = "transport is closing"  system=system
INFO[0075] pickfirstBalancer: HandleSubConnStateChange: 0xc00015e200, TRANSIENT_FAILURE  system=system
INFO[0075] pickfirstBalancer: HandleSubConnStateChange: 0xc00015e200, CONNECTING  system=system
WARN[0075] finished streaming call with code Unavailable  error="rpc error: code = Unavailable desc = transport is closing" grpc.code=Unavailable grpc.method=Method grpc.service=namespace.Service grpc.start_time="2019-10-28T11:56:35Z" grpc.time_ms=1052.424 span.kind=server system=grpc

[johanbrandhorst]

Now we begin the delicate dance of making the CI pass...

[johanbrandhorst]

Thanks for the fix @crlssn!