Replace can_start() with can_prepare()

[asvetlov]

Fix for #2789

[codecov-io]

Codecov Report

Merging #2794 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2794   +/-   ##
=======================================
  Coverage   97.99%   97.99%           
=======================================
  Files          39       39           
  Lines        7379     7379           
  Branches     1296     1296           
=======================================
  Hits         7231     7231           
  Misses         47       47           
  Partials      101      101

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[new issue]: https://github.com/aio-libs/aiohttp/issues/new