"is not a legal HTTP header name"

**Intended outcome:**

```shell
npx apollo service:download \
--endpoint=https://asdf.url/graphql \
--header="X-APP-TOKEN: asdf" \
--header="X-AUTH-TOKEN: asdf" \
graphql-schema.json
```

should download the schema

**Actual outcome:**

```
✔ Loading Apollo Project
✖ Saving schema to schema.json
→ TypeError: is not a legal HTTP header name
Error: TypeError: is not a legal HTTP header name
```

**How to reproduce the issue:**

run code above ↑

**Versions**

`apollo/2.33.2 darwin-x64 node-v16.0.0`

Start with the Apollo CLI service:download entry point and reproduce the command using the two --header options shown in the issue. Trace how those options are parsed and passed into the HTTP request, then verify that the schema downloads successfully without the illegal-header-name error.

Written by the indexing model from the issue text.