Update http-signature version.

Going from the 0.9 to 0.10 series includes a protocol incompatible change. The change was made by me in the spec and node code due to a security issue: https://github.com/joyent/node-http-signature/issues/10. Considering how new HTTP Signature support is and the limited use of this spec I think it's safe to potentially cause breakage for any limited users of this feature.