Handle `httpx` metadata in the `requre-purge`

[mfocko]

As opposed to the requests, httpx has a bit different naming for some of the attributes that are purged…

The main motivation for purging those attributes comes from the fact that they usually change with each of the regeneration of the data which also contributes to the diff…

Namely:

• requests.sessions%send:Date:str → httpx._client%send:date:str
• requests.sessions%send:elapsed → httpx._client%send:_elapsed:float

Looking at the naming of the attributes, I'm not sure how robust this “replacing” is and whether it's possible to corrupt the saved responses by purging… However I guess that's out of the scope of this issue.