fwdm: fix session proxying of SSL sockets, and documentation

[gpotter2]

• Improvements to forwarding machine to support cases with SSL sockets
• fix HEAD packets without Content-Length in TCP reassembly
• HTTP: fix re-build of payload with chunks

[codecov]

Codecov Report

❌ Patch coverage is 84.21053% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.35%. Comparing base (929066f) to head (99fafd9).

Files with missing lines	Patch %	Lines
scapy/fwdmachine.py	0.00%	2 Missing ⚠️
scapy/supersocket.py	83.33%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #5016   +/-   ##
=======================================
  Coverage   80.35%   80.35%           
=======================================
  Files         386      386           
  Lines       95983    95996   +13     
=======================================
+ Hits        77125    77141   +16     
+ Misses      18858    18855    -3     

Files with missing lines	Coverage Δ
scapy/layers/http.py	83.33% <100.00%> (+0.18%)	⬆️
scapy/supersocket.py	76.71% <83.33%> (+0.11%)	⬆️
scapy/fwdmachine.py	0.00% <0.00%> (ø)

... and 5 files with indirect coverage changes

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