feat(graphql): forces http header value to be lowercase

[jeremyfiel]

While RFC2616 indicates http header field names are case insensitive, it leaves http header field values more open to interpretation, such as using the text

Applications ought to follow "common form", where one is known or
indicated, when generating HTTP constructs, since there might exist some implementations that fail to accept anything beyond the common forms.

This is a small user experience improvement for REST Client users to prevent a negative experience by using a case sensitive value.

This is also consistent with the current code snippet for getHeader method which forces lowercase on the http header fields.

vscode-restclient/src/utils/misc.ts

Line 12 in 80ee7d1

const headerName = Object.keys(headers).find(h => h.toLowerCase() === name.toLowerCase());

fixes #1236