Upcoming changes to HTTP header casing

Infrastructural changes to BigCommerce over the next 2 weeks may result in some HTTP headers being returned as lowercase which previously had a mixture of uppercase and lowercase characters.

For example, HTTP header names such as X-Rate-Limit-Requests-Left may instead be returned as x-rate-limit-requests-left. Per the HTTP specification (https://www.w3.org/Protocols/rfc2616/rfc2616-sec4.html#sec4.2), HTTP header names should always be treated as case-insensitive, so we do not consider this to be a breaking change to the API. Most HTTP clients already treat headers with the appropriate case insensitivity, so we expect minimal customer impact from this change.

No changes are being made to HTTP header values as part of this change.