OCSP stapling: check Content-Type.

OCSP stapling: check Content-Type.

This will result in better error message in case of incorrect response
from OCSP responder:

... OCSP responder sent invalid "Content-Type" header: "text/plain"
    while requesting certificate status, responder: ...

vs.

... d2i_OCSP_RESPONSE() failed (SSL:
    error:0D07209B:asn1 encoding routines:ASN1_get_object:too long
    error:0D068066:asn1 encoding routines:ASN1_CHECK_TLEN:bad object header
    error:0D07803A:asn1 encoding routines:ASN1_ITEM_EX_D2I:nested asn1 error)
    while requesting certificate status, responder: ...