Support nested usage of JsonApiClient::Resource.with_headers

[tom-lord]

The problem:

When the .with_headers block closes, it wipes any headers that have been set. This can lead to counterintuitive issues in code; consider:

def my_record
  JsonApiClient::Resource.with_headers("Authorization: ...") do
    # fetch data
  end
end

def update_record
  JsonApiClient::Resource.with_headers("Authorization: ...") do
    my_record.save
  end
end

Regardless of whether the headers inside both blocks are the same or different, the JsonApiClient "forgets" them, halfway through the block! This causes my_record.save to fail as an unauthorised request.

The current workaround might be to instead do:

def my_record
  JsonApiClient::Resource.with_headers("Authorization: ...") do
    # fetch data
  end
end

def update_record
  record = my_record
  JsonApiClient::Resource.with_headers("Authorization: ...") do
    record.save
  end
end

...But this is confusing, and cumbersome.

My proposal in this PR is to "reset" the headers to their previous state at the end of a .with_headers block, rather than wiping them completely. So the first code block above will work as expected.

[tom-lord]

CC: @gaorlov