Fix streaming with httpx (fixes #597)

[parkerhancock]

Fixes #597 by having VCR work when the httpx streaming interface is used.

[codecov-commenter]

Codecov Report

Merging #741 (d6c25e4) into master (c95b726) will decrease coverage by 0.10%. The diff coverage is 100.00%.

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@@            Coverage Diff             @@
##           master     #741      +/-   ##
==========================================
- Coverage   90.63%   90.54%   -0.10%     
==========================================
  Files          28       28              
  Lines        1794     1798       +4     
  Branches      268      322      +54     
==========================================
+ Hits         1626     1628       +2     
- Misses        133      135       +2     
  Partials       35       35              

Impacted Files	Coverage Δ
vcr/stubs/httpx_stubs.py	96.03% <100.00%> (-1.90%)	:arrow_down:

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[hartwork]

@parkerhancock any news on the next iteration? I'd be happy to do review.

[hartwork]

@parkerhancock any news on the next iteration? I'd be happy to do review.

[hartwork]

@parkerhancock any news on the next iteration? I'd be happy to do review.

[parkerhancock]

Update here - I'm really stuck on this. For some reason, vcrpy "works" with the streaming interface (i.e. it caches something and returns something), but it only caches the first chunk, not the whole request. Not sure where to go from here.

[parkerhancock]

See #784, which fixes this and all other HTTPX issues.