Better handling of objects with multiple equal keys.

[madeso]

The current implementation of the object uses a std::map for storing the key/values but according to ecma 404 there can be many entries with the same key.

There should be at least 3 options:

• All but the last key is ignored (current behavior)
• Multiple equal keys are an error
• All keys are stored in a multimap.

[madeso]

Given commit d99754a the 2 top functions are now possible, the only thing that remain is the ability to store keys in a multimap.

[madeso]

Reading multikeys are not important, in the new "opinionated json parser", and ignored and error are already supported, so I'm closing this issue since it's complete.