Adding content-type header is not supported when fetching schema

[felamaslen]

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Issue workflow progress

Progress of the issue based on the Contributor Workflow

• [x] 1. The issue provides a reproduction available on Github, Stackblitz or CodeSandbox

  Make sure to fork this template and run yarn generate in the terminal.

  Please make sure the GraphQL Tools package versions under package.json matches yours.

• [x] 2. A failing test has been provided
• [x] 3. A local solution has been provided
• [x] 4. A pull request is pending review

---

Describe the bug

To Reproduce Steps to reproduce the behavior:

• Run graphql-codegen on a schema like the following:

schema:
  - https://my.schema/graphql:
      headers:
        content-type: 'application/vnd.api+json'
extensions:
  codegen:
    generates:
      'schema.graphql':
        plugins:
          - schema-ast

• Observe that the request for introspection is made with header content-type: application/json, i.e. your config is not obeyed

Expected behavior

• The request for introspection is made with the header content-type: application/vnd.api+json

Environment:

• OS:
• @graphql-tools/executor-http: 1.0.6
• NodeJS:

Additional context

The bug is caused by this line: https://github.com/ardatan/graphql-tools/blob/master/packages/executors/http/src/index.ts#L204