OAuthAccessTokenView uses "body" signature type, instead of "header"

[Dimitri John Ledkov]

As per https://help.launchpad.net/API/SigningRequests launchpad uses "header" signature type only.
For all requests, but "OAuthAccessTokenView" (exchanging request token for an access one) which force uses "body" signature type. As far as i can tell, per-spec, server is suppose to reply via body yet request can come in any form.

Ideally launchpad would use "header" signature type here.

And for backwards compatibility continue accept "body" signature type for this request only.