fix(ngResource): don't filter "$"-prefixed properties from ngResource requests/responses

ngResource no longer filters properties prefixed with a single "$" character from requests or
responses, correcting a regression introduced in 1.2.6 (cb29632a) which caused shallowCopy and
shallowClearAndCopy to ignore properties prefixed with a single "$".

Closes #5666
Closes #6080
Closes #6033