Fix SocketClient to close subscription on socket close

[qbx2]

Closes #1183

[codecov[bot]]

Codecov Report

Merging #1184 (24e9678) into main (0d8a3d0) will decrease coverage by 0.10%. The diff coverage is 68.75%.

@@            Coverage Diff             @@
##             main    #1184      +/-   ##
==========================================
- Coverage   59.72%   59.61%   -0.11%     
==========================================
  Files          41       41              
  Lines        1676     1684       +8     
==========================================
+ Hits         1001     1004       +3     
- Misses        675      680       +5     

Impacted Files	Coverage Δ
...l/lib/src/links/websocket_link/websocket_link.dart	0.00% <ø> (ø)
...lib/src/links/websocket_link/websocket_client.dart	80.47% <68.75%> (-1.71%)	:arrow_down:

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[qbx2]

I’ll try

[qbx2]

I added a test for onDone(). However, I have no idea to test onError().

[vincenzopalazzo]

I'm too lazy to fix the git history and the commit message! So I just cherry picked your commit https://github.com/zino-hofmann/graphql-flutter/pull/1186