Handle float/int slice problem and null values

[compscidr]

Based on findings outlined here: #14 (comment)

I think this will handle the case where we values which are mostly ints, but we get a float, or we have mostly floats but get an int (this is possible because firebase will change floats which are even to ints, for instance if you try to save 400.0, firebase will save it as 400, and if all the other values in a collection are floats, we will have problems trying to append to the slice with the existing code.

I expect adding a null into the mix will cause a similar crash, so in this case, I just detected a nil and added a 0 or 0.0 depending on the slice type.

I'm not entirely sure there isn't also a problem in the fireql library that this uses as well - will try out to verify asap.

[pgollangi]

LGTM! Thanks a lot @compscidr for the contribution.