Allow disabling the 'Accept-Encoding' header

[COM8]

Resolves #925

Adds a new value for AcceptEncoding: disabled. When a user passes this value to a AcceptEncoding object it will prevent curl from creating a Accept-Encoding.

It has the same behaviour as passing NULL to the curl option CURLOPT_ACCEPT_ENCODING.

When passing multiple values to AcceptEncoding including disabled, an exception of type std::invalid_argument get thrown when performing a request.

Examples

cpr::Session session;
session.SetUrl("https://example.com");
session.SetAcceptEncoding({AcceptEncodingMethods::disabled}); // Disable setting the `Accept-Encoding` headder
Response response = session.Get();

cpr::Session session;
session.SetUrl("https://example.com");
session.SetAcceptEncoding({"disabled"}); // Disable setting the `Accept-Encoding` headder
Response response = session.Get();

cpr::Session session;
session.SetUrl("https://example.com");
session.SetAcceptEncoding({AcceptEncodingMethods::disabled, AcceptEncodingMethods::deflate});
Response response = session.Get(); // An exception of type  `std::invalid_argument` will be thrown here since multiple values are passed to `AcceptEncoding` where one of them is `disabled`

[obelixzgz]

It's perfect.

[obelixzgz]

Thank you very much

[COM8]

Perfect. Will finish up this PR next weekend and add docs for it.

[COM8]

Docs: libcpr/docs#40