set_mount_point accepts HTTP headers

Add `cache_control` parameter to `set_mount_point` (#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

Fix for '100 Continue' problem on client

v0.7.8

Fix #661

Added CPPHTTPLIB_COMPRESSION_BUFSIZ

Add compression buffer size customization (#644)

* add compression buffer size customization and small brotli refactor

* allocat brotli buffer once

* add init to brotli decoder buffer

Fixed various bugs

v0.7.6

Fix server crash caused due to regex complexity while matching header…

Content provider without content length

v0.7.5

Update README

Brotli support on server

v0.7.4

Fixed example build errors

KeepAlive problem with a read time out client request

v0.7.3

Fix #591

Brotli compression support on client

Add Brotli Cmake support (#584)

Had to create a custom FindBrotli package, as not all users have
PkgConfig installed (which Brotli uses). This file gets installed
alongside httplibConfig.cmake for the end-users convenience.

Set BROTLI_USE_STATIC_LIBS to ON if you want to find the static libs
instead of default shared.

Adds the HTTPLIB_REQUIRE_BROTLI (default off) and HTTPLIB_USE_BROTLI_IF_AVAILABLE
(default on) options, which work in the same manner as the other optional/required
dependency options.

Moved the scattered linking and definitions to a single call.

Updated some documentation about the new options.

Improved the in-tree support by setting the HTTPLIB_IS_USING_XYZ
variables in the main CMakeLists (as well as having them in the
httplibConfig.cmake file).

Fixes #582

Use the default ca bundle in system

v0.7.1

Fix #566

Better support for Keep-alive on the client side

New Keep-Alive support on Client which supersedes the old batch style Keep-Alive API.