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

std::tolower is undefined if the argument's value is neither represen…

…table as unsigned char nor equal to EOF (yhirose#761)

Co-authored-by: taoxu <[email protected]>

Fix yhirose#738

Fix problem with invalid range

Fix yhirose#731