Pack extension requests in a single sequence

The ASN.1 description of extension requests could be incorrectly read
that extensions should be packed in individual sequences, while in fact
the extensions themselves are sequences and should be all packed in a
single sequence. This commit fixes how CSRs create and return extension
requests.
Merge pull request #2101 from adrienthebo/defect/csr-extreq-pack-single-seq

Pack extension requests in a single sequence