Allow lax parsing of keyUsage bit string

[jschanck]

This certificate has a non-zero padding bit in its keyUsage extension and it has been logged by several CT logs.

It would be useful if the asn1 parser in this library allowed lax parsing of bit strings. In particular, lax parsing should allow valid BER encodings with non-zero padding bits. So

certificate-transparency-go/asn1/asn1.go

Line 235 in 5162ff6

bytes[len(bytes)-1]&((1<<bytes[0])-1) != 0 {

should be something like

 (!lax && bytes[len(bytes)-1]&((1<<bytes[0])-1) != 0)