Remove ineffective/unneeded const from shared_ptr return types

[stuart-byma]

Identify the Bug

#284

Description of the Change

Replace all instances of const shared_ptr<http_response> with shared_ptr<http_response>

Alternate Designs

There is no alternate design

Possible Drawbacks

I don't believe there are any.

Verification Process

All tests pass. Everything compiles fine. clang-tidy no longer reports the issue in a code base that uses this return type.

Release Notes

Remove ineffective/unneeded const from std::shared_ptr <http_response> return types.

[etr]

Thanks for the cleanup. Very much appreciated

[jcphill]

I know it's a bit late but this should have been flagged as a breaking interface change because existing code that overrides httpserver::http_resource::render_GET etc. now has an "invalid covariant return type" error and there is no way to write code that works with both the old const and new non-const return types.