Add move semantic for httplib::Result::res_::body

[Bulb4]

Currently we need to copy body's value, because all httplib::Result getters are const. There is also no way to get httplib::Result::_res ownership because it is private and unique_ptr.
It would be nice to by able to use std::move for body, if we want to store it somewhere, and use after result destruction.

[yhirose]

@sum01, do you have any thought? I don't mind making the getters back to non-const. #613

[sum01]

I still think it makes sense to be const-qualified since the methods don't change anything, which is why I originally made that issue.

Does const_cast not solve your issue @Bulb4 ?
Either that, or maybe add a method to return the shared_ptr<Response> res_ like get_res_ptr or something.

EDIT: or alternativenly add duplicate getters that are all non-const, like

const Response &value() const { return *res_; }
Response & value() { return *res_; }
const Response &operator*() const { return *res_; }
Response &operator*() { return *res_; }
const Response *operator->() const { return res_.get(); }
Response *operator->() { return res_.get(); }

[yhirose]

@sum01, after I carefully see the current code, those methods shouldn't be changed as you mentioned. Sorry for my hasty comment.

Your suggestion "or alternatively add duplicate getters that are all non-const, like" sounds good. I'll go ahead to do so.