openpgp: pass hash to Signer.Sign for ECDSA
openpgp: pass hash to Signer.Sign for ECDSA

Change-Id: Iabb601d9d7f3394c2a20cacd042c00bd05457500
Reviewed-on: https://go-review.googlesource.com/c/137897
Reviewed-by: Filippo Valsorda <4f40cda291c5f9634e1affd3db44947af61f705c@golang.org>
Run-TryBot: Filippo Valsorda <4f40cda291c5f9634e1affd3db44947af61f705c@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

golang/crypto,golang/crypto,golang/crypto