ocsp: better validate OCSP response's certificates #256
      
        
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We make three changes here:
Previously, when a CA returned the intermediate CA that signed a leaf cert as an additional cert in the response field (without using a delegated OCSP certificate), Go would err with a bad signature, as it expected the intermediate CA to have signed the wire copy (even though it was the exact same certificate).
Also includes a code comment around the "bad signature on embedded certificate" error message, indicating that this isn't strictly the correct preposition choice.
See also: https://github.com/crtsh/test_websites_monitor/blob/1bd8226b5f963e91d7889ea432a36e3173be8eec/test_websites_monitor.go#L267
See also: golang/go#59641
Fixes golang/go#59641