Content-Type header is case sensitive

[shazil-arif]

If the 'Content-Type' header is not set exactly, nrc-parser-manager throws a JSON parse error. For example, setting the header as 'Content-type' throws an error

According to https://www.w3.org/Protocols/rfc1341/4_Content-Type.html#z0 the Content-Type header should not be case sensitive so this should be considered.