sign_request -> build_auth_headers

[richvdh]

I just got very confused about the fact that the headers are only an output, not an input.

[codecov-io]

Codecov Report

Merging #4408 into develop will increase coverage by <.01%.
The diff coverage is 71.42%.

@@             Coverage Diff             @@
##           develop    #4408      +/-   ##
===========================================
+ Coverage    73.65%   73.66%   +<.01%     
===========================================
  Files          300      300              
  Lines        29815    29816       +1     
  Branches      4897     4897              
===========================================
+ Hits         21961    21964       +3     
+ Misses        6412     6411       -1     
+ Partials      1442     1441       -1

Impacted Files	Coverage Δ
synapse/handlers/identity.py	26.47% <0%> (ø)	⬆️
synapse/http/matrixfederationclient.py	84.09% <100%> (+0.06%)	⬆️
synapse/handlers/user_directory.py	71.08% <0%> (-0.31%)	⬇️
synapse/handlers/device.py	81.45% <0%> (+0.8%)	⬆️
synapse/util/file_consumer.py	82.45% <0%> (+1.75%)	⬆️

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