fix file descriptor leak problem & enable keepalive by default

[plpan]

No description provided.

[pkopac]

@QuentinPerez: IDK about the keep-alive, but the resp.Body.Close() is certainly not correct in current code; it only gets invoked on the NopCloser, which... does nothing. So, the Reader from http stays open forever.

[plpan]

@pkopac When the resp.Body is closed, we can fix leak by reusing the obj of SuperAgent. We have proved it works.

[pkopac]

@plpan: thanks, makes sense... but, if you don't reuse all instances (eg. we don't in our code), then it leaks. So I still consider this a bug.

[plpan]

@pkopac Yeal, you're right. We should disable it by default.