Default headers in `@graphql-tools/url-loader` are case sensitive

### Issue workflow progress

_Progress of the issue based on the [Contributor Workflow](https://github.com/the-guild-org/Stack/blob/master/CONTRIBUTING.md#a-typical-contributor-workflow)_

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

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**Describe the bug**

Default headers (`accept` and `content-type`) for `UrlLoader` are set with the keys in lower case. If you try to override them but use capitalized keys, the headers get duplicated. For example, if I set the header the way it is [demonstrated in the documentation](https://github.com/ardatan/graphql-tools/blob/026f0d7afad120fb599e5f10b66839091f7dacbf/website/docs/schema-loading.mdx#url-loader), I send both `accept` and `Accept` headers. The same problem occurs with `content-type` header.

For me this produced a bag with introspection of this endpoint—`https://api.spacex.land/graphql`

**To Reproduce**
Steps to reproduce the behavior:
1. Create a mock `customFetch` function to capture headers to be send
2. Override default headers with non-lowercase keys to the loader's `load` method
3. Provide the mock function to the loader's `load` method
4. Log the headers

[Sandbox with reproduced error](https://stackblitz.com/edit/node-5vz71l)

How the error was found:
1. Provide the loader's `load` function with url `https://api.spacex.land/graphql` and set the `Set-Content` header to default value of `application/json`
2. Catch the error
3. Comment previously set header or make it lowercase and you won't receive the error

[Sandbox with reproduction](https://stackblitz.com/edit/node-nlfnyv)

**Expected behavior**
1. If the headers with internally set default values are overridden, incoming headers are made lowercase before being applied.
2. There is some information provided in the documentation about those 2 headers being set by default, so people won't unnecessarily override them with the same values.

**Environment:**

- OS: MacOS 12.3.1
- `@graphql-tools/url-loader`:
- NodeJS: 17.3.0

**Additional context**

Start at the @graphql-tools/url-loader load entry point and trace how its default headers are combined with incoming headers, using the two linked StackBlitz reproductions to observe the result. Check website/docs/schema-loading.mdx for the documented defaults; done means overridden headers are not duplicated and the documentation explains the defaults.

Written by the indexing model from the issue text.