Create secret - case insensitive Content-Type check

Allows for upper case and mixed case types, subtypes, and parameters
in the Content-Type header field as per RFC 1341. More details can
be found at http://www.w3.org/Protocols/rfc1341/4_Content-Type.html.

Closes-Bug: #1252453
Change-Id: I400a05593e0c622de93a47a0fe0394793eb777f1