Group function alias names

[octaviancorlade]

The current output when an aggregate function is used multiple times in a query looks like this and makes it impossible to distinguish the columns

[
    {
        "column1": "abc",
        "sum": 10,
        "sum": 20,
    }
]

This enables the syntax _select=column1,sum:column2:alias2,sum:column3:alias3&_groupby=column1 to return

[
    {
        "column1": "abc",
        "alias2": 10,
        "alias3": 20,
    }
]

Also related to prest/prest#216

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[tiaguinho]

@avelino can you check?