Hurl debug curl command is erroneous with text request body that start with `@`

[jcamiel]

Given this Hurl file:

POST https://httpbin.io/anything
```
@filename
```

(post a text body @filename), the debug curl command produced by Hurl is erroneous:

$ hurl --verbose test.hurl
* ------------------------------------------------------------------------------
* Executing entry 1
*
...
* Request can be run with the following curl command:
* curl --header 'Content-Type:' --data $'@filename\n' 'https://httpbin.io/anything'
*
> POST /anything HTTP/1.1

Then running the curl command:

$ curl --header 'Content-Type:' --data $'@filename\n' 'https://httpbin.io/anything'
curl: Failed to open filename

curl: option --data: error encountered when reading a file
curl: try 'curl --help' or 'curl --manual' for more information

When producing curl command, we should:

• use --data-raw for non binary body that doesn't start with an @
• use --data-binary otherwise