Fix httpx upload

[07pepa]

• removed unneeded decoding of body in httpx_stubs.py
• added asset to upload
• added equivalent cassette for upload
• added testing to cover uploading to vcr with httpx
• fixes https://github.com/kevin1024/vcrpy/issues/656

[parkerhancock]

Fixed in #784

[parkerhancock]

I suggest we close this one in view of #784