Fix web.run_app not to bind to default host:port if only socket is passed

[popravich]

What do these changes do?

web.run_app fix to create server only for specified socket if no other arguments are passed.
Currently, run_app will also bind to default host if only socket is passed which is wrong.

Are there changes in behavior for the user?

no

Checklist

• I think the code is well written
• Unit tests for the changes exist
• Documentation reflects the changes
• If you provide code modification, please add yourself to CONTRIBUTORS.txt
  • The format is <Name> <Surname>.
  • Please keep alphabetical order, the file is sorted by names.
• Add a new entry to CHANGES.rst
  • Choose any open position to avoid merge conflicts with other PRs.
  • Add a link to the issue you are fixing (if any) using #issue_number format at the end of changelog message. Use Pull Request number if there are no issues for PR or PR covers the issue only partially.

[popravich]

cc: @fafhrd91, @kxepal

[kxepal]

LGFM

[fafhrd91]

@popravich I am planing to release 2.0.6, if you want to add this to it, just cherry pick you changes into 2.0 branch

[popravich]

ok, will do

[popravich]

done

[fafhrd91]

thanks

[lock]

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