According to RFC 2616 Section 4.2, HTTP headers are case-insensitive. It is up to the HTTP client to send "Content-Type" or "content-type". For example, net/http will send the header as 'Content-Type'.
However, there are some services that do not follow this rule and insist on a specific case. E.g. all lowest case. As a walkaround for this issue, we will create a subclass of string and override the capitalize method.
This will prevent HTTParty (which uses net/http underlying) from capitalizing the header.