Fix controlled closing of websockets

[rowanseymour]

Addresses #109

[codecov]

Codecov Report

Attention: 7 lines in your changes are missing coverage. Please review.

Comparison is base (6fc7181) 89.72% compared to head (6daeb9a) 90.16%.

Files	Patch %	Lines
httpx/websocket.go	85.71%	6 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #110      +/-   ##
==========================================
+ Coverage   89.72%   90.16%   +0.43%     
==========================================
  Files          39       39              
  Lines        1859     1891      +32     
==========================================
+ Hits         1668     1705      +37     
+ Misses        153      150       -3     
+ Partials       38       36       -2     

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