Not all `operationHeaders` are forwarded

### Issue workflow progress

_Progress of the issue based on the
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- [ ] 1. The issue provides a reproduction available on
[Github](https://github.com/Urigo/graphql-mesh/tree/master/examples/hello-world),
[Stackblitz](https://stackblitz.com/github/Urigo/graphql-mesh/tree/master/examples/hello-world)
or
[CodeSandbox](https://codesandbox.io/s/github/Urigo/graphql-mesh/tree/master/examples/hello-world)

> Make sure to fork this template and run `yarn generate` in the terminal.
>
> Please make sure Mesh 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

---

**Describe the bug**

I recently needed to forward a new custom header by listing it under the `operationHeaders` section in configuration file.

**To Reproduce** Steps to reproduce the behavior:

It's basically nothing special, I just added a new header in configuration file. Here is the meshrc.yaml
```
sources:
- name: CmsApi
handler:
graphql:
endpoint: ${SOME_BASE_URL}/graphql
operationHeaders:
"x-app-name": "{context.headers['x-app-name']}"
"x-app-platform": "{context.headers['x-app-platform']}"

- name: UserApi
handler:
graphql:
endpoint: ${ANOTHER_GQL_API}/graphql
operationHeaders:
Authorization: "{context.headers.authorization}"
"auth-provider": "{context.headers['auth-provider']}"
"x-app-name": "{context.headers['x-app-name']}"
"x-app-platform": "{context.headers['x-app-platform']}"

- name: XdrApi
handler:
jsonSchema:
endpoint: ${XDR_API_BASE_URL}
operations:
- type: Query
field: xdrData
path: /xdr/playhead/account/{args.someArg}/{args.someArg2}
method: GET
responseSchema: ./xdr/json-schemas/playheadData.json
# responseTypeName: xdrData

codegen:
#Default values can be found here https://the-guild.dev/graphql/mesh/docs/guides/graphql-code-generator
skipTypename: true
flattenGeneratedTypes: false
onlyOperationTypes: false
preResolveTypes: false
namingConvention: keep
documentMode: graphQLTag
gqlImport: "@graphql-mesh/utils#gql"
enumsAsTypes: true
ignoreEnumValuesFromSchema: true
useIndexSignature: true
contextType: MeshContext
```
And below is what I got on server side

**Expected behavior**

Obtaining the defined new `x-app-platform` value.

**Environment:**

- OS: Cloudflare Worker
- `@graphql-mesh/...`: `"@graphql-mesh/graphql": "^0.95.2"`
- NodeJS:

**Additional context**

Start by reproducing the configuration shown in meshrc.yaml, using the GraphQL Mesh GraphQL handler and the Cloudflare Worker environment. Trace how operationHeaders are forwarded to the upstream APIs, checking why x-app-platform is omitted while the other configured headers arrive. Done means the configured x-app-platform value reaches the upstream server.

Written by the indexing model from the issue text.