Replaced return type to auto.

[redboltz]

No description provided.

[codecov-commenter]

Codecov Report

Attention: 5 lines in your changes are missing coverage. Please review.

Comparison is base (c7a5326) 88.88% compared to head (257b4fd) 88.89%.
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #110   +/-   ##
=======================================
  Coverage   88.88%   88.89%           
=======================================
  Files          95       95           
  Lines       10249    10256    +7     
=======================================
+ Hits         9110     9117    +7     
  Misses       1139     1139           

Files	Coverage Δ
include/async_mqtt/broker/endpoint_variant.hpp	98.31% <ø> (ø)
include/async_mqtt/endpoint.hpp	83.79% <100.00%> (ø)
include/async_mqtt/ws_fixed_size_async_read.hpp	96.29% <ø> (ø)
include/async_mqtt/stream.hpp	82.25% <87.80%> (+0.34%)	⬆️