fix filter composition

[autre]

This should close #42.

[mcorbe]

Quick observation, this fix wont work if you nest "or" and "and" (basically would only return last bitwise operator).

You need to check for the actual type as such :

``
def and(self, x):

   if 'type' in self.filter['filter'] and self.filter['filter']['type'] == "and":
        flattened = self.flatten()
        flattened.extend(x.flatten())

        return Filter(type="and", fields=flattened)

    return Filter(type="and",
                  fields=[self.filter['filter'], x.filter['filter']])

``

[xvrl]

this seems to be similar to #46. @autre can you work with @dakra to merge your two code bases and or decide which one to use?

[autre]

@xvrl: from a quick look, seems like @dakra's pr is pretty similar (and maybe simpler/cleaner). Moreover it handles the case for more than one and/or's at filter construction time. If you think #46 is good to land, I'm ok with closing this one.

[xvrl]

@autre #46 was merged, so feel free to close this one.