Fix Aggregation Web formatter

[jal347]

Histogram aggregations do not contain the fields aggregations.<term>.doc_count_error_upper_bound, aggregations.<term>.sum_other_doc_count

In our es formatter, it assumes that these two values exist. https://github.com/biothings/biothings.api/blob/master/biothings/web/query/formatter.py#L426-L427

When making a custom aggregation in the esquerybuilder, I have to override these two values.

res[facet]['other'] = res[facet].pop('sum_other_doc_count', 0)
res[facet]['missing'] = res[facet].pop('doc_count_error_upper_bound', 0)

We should have this as a quick fix so when other users make custom aggregations they wont have to override the transform_aggs method.