Make x-amz-access-token Header Case-Insensitive

[JordanForeman]

According to the HTTP/1.1 spec:

Each header field consists of a name followed by a colon (":") and the field value. Field names are case-insensitive. [src]

After a bit of poking and prodding, as well as some internet sleuthing, it would seem that the SP-API does validate the case of x-amz-access-token, and that it enforces lowercasing.

Generally speaking this seems unnecessary, and has led to much confusion online. However it is almost unworkable in the Ruby community due to the capitalization enforced by the Net::HTTPHeader module, and documented in this very repository.

Needless to say that it would be tremendously beneficial from a developer's standpoint to not have to worry about header name casing.

See also: https://github.com/amzn/selling-partner-api-docs/issues/292