Preserve schema key order when flattening a dict grouping

[jonmmease]

Small follow-on fix for flexible callback support added in #1691.

For the case of a callback that returns a dictionary grouping, this fix removes the unintentional requirement that the dictionary key order of the returned value match the key order of the dependency specification.

For example, this case now works properly:

@app.callback(dict(a=Output(...), b=Output(...)), ...)
def callback(...):
    return dict(b=0, a=1)

[alexcjohnson]

Nice catch! And bonus points for reducing LOC (aside from tests) with the fix!
💃