Prevent body from being closed by the Request RoundTripper

[zekroTJA]

Because the Request RoundTripper closes passed body streams if they implement io.Close, seeking the body on re-trying the request after an authentication error fails because the body stream is already closed.

This implementation prevents the RoudTripper from closing the body so we can postpone the closing until we are done with the body stream.

[chripo]

i think this could lead to multiple close calls / undefined behavior because we do recursion due to authentication errors.

[chripo]

#15 is a long thorn in my side :)

[chripo]

I've made some progress on #15 this will hopefully close this issue too.

[chripo]

if the issue still present, we could continue on top of master.

[ueffel]

btw: there is already io.NopCloser in the stdlib. No need to reinvent it :)