Additional changes for yhirose#889

Fix yhirose#878

Fix yhirose#860

Add exception handler (yhirose#845)

* Add exception handler

* revert content reader changes

* Add test for and fix exception handler

* Fix warning in test

* Readd exception test, improve readme note, don't rethrow errors, remove exception handler response

Resolve yhirose#839

No content check (yhirose#823)

* No content check

* unit test for no content

* fixing merge conflict break

* oops during manual merge conflict

Updated the simple example

Fixed problem with invalid requests including spaces in URL path

Removed `std::atomic<Error> error_`

Fixed build errors and apply clangformat