Headers should be case insensitive

When adding a content-type and setting the request as POST, it duplicates the content-type header:

```
http +profile '/api/v1/lists?api_key={privateKey}' -X POST list_name=test -H 'Content-type:application/x-www-form-urlencoded'
```

Output:

```
Content-Type: application/json
Content-type: application/x-www-form-urlencoded
....
```