bugfix: don't change the object passed in the constructor

[tiagogoncalves89]

Now, the structure never change any data outside the library escope.

[wenderjean]

@tiagogoncalves89 Would you mind to elaborate better about the problem itself?

[talyssonoc]

@wenderjean I think what he meant is that, currently, we mutate the attributes object passed to the structure constructor.

@tiagogoncalves89 can you use object spread instead of Object.assign? Also do you think we could change the word "change" for "mutate" to make it use a more standard word?

[talyssonoc]

Closed by #77