json_pointer<>::back() should be const

[gcerretani]

• What is the issue you have?
  It is impossible su use json_pointer<>::back() on const json_pointer.

• What is the expected behavior?
  I expect to be able to use on a const. Since it returns a const std::string&, and since it is implemented with std::vector<>::back() that has a const overload, I expect the json_pointer<>::back() has to be declared const.

• And what is the actual behavior instead?
  The function is not marked as const.

• Did you use a released version of the library or the version from the develop branch?
  tested with 3.7.0.

[nlohmann]

Thanks for reporting! I think this can be indeed easily fixed. I'll make a PR.

[gcerretani]

In the release notes of #v3.7.1 release v3.7.1, the fix is not correctly described:

• Added const version for json_pointer::back.

The link points to basic_json::back instead of json_pointer::back. Furthermore, the version const has not been added, it just replaces the old non const version.

[nlohmann]

Good catch! I fixed the release notes accordingly.