Content-Type response header detection should not be case sensitive

[vch-7]

Header field names and MIME type detection should not be case sensitive.
The getResult function checks for the existence of the Content-Type response header. This check should be able to accept a case-insensitive header field name and value (for e.g. content-type: application/json)

function getResult(response: Dom.Response): Promise<any> {
  const contentType = response.headers.get('Content-Type')
  if (contentType && contentType.startsWith('application/json')) {
    return response.json()
  } else {
    return response.text()
  }
}

The primary use case for this would be when the default fetch config in the GraphQLClient instance is overridden with say, an Axios wrapper, where the Response Header object does not implement a case-insensitive get method by default.

References:
https://www.rfc-editor.org/rfc/rfc7540#section-8.1.2
https://www.rfc-editor.org/rfc/rfc6838.html#section-4.2