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 <taoxu@bilibili.com>

Fix yhirose#738

Fix problem with invalid range

Fix yhirose#731

Fixed compiler error for old compiler.

Fix yhirose#706

Add `cache_control` parameter to `set_mount_point` (yhirose#688)

* Add `cache_control` parameter to `set_mount_point`

Specifies the Cache-Control header value to return when specified. For example:

```
svr.set_mount_point("/assets", "public/assets", "public, max-age=604800, immutable");
```

* Add default for cache_control

Default to "no-cache", which is implicitly what is happening today.

* Change set_mount_point to accept Headers

* Don't use C++17 destructuring