Send date_histogram extended_bounds as a long. Closes #1868

[rashidkpc]

Fixes extended_bounds in date_histogram agg for for users that have non-ISO8601 timestamps

[w33ble]

Seems right to me based on the issue it's closing and the parent of that. I'm not super familiar with extended_bounds though, and the guide doesn't make it clear what the min and max values may end up being. That said, I'm going to ask that someone else who does know this review as well.

And, one comment on the test - it's worth passing in values for min and max instead of just the moment object, as I believe that's more the "norm" (based on the docs). Making a test like so might be useful:

describe('extended_bounds', function () {
        it('should write the long value of the moment passed in', function () {
          var min = 0;
          var max = 500;
          var output = paramWriter.write({
            extended_bounds: {
              min: min,
              max: max
            }
          });
          expect(output.params.extended_bounds.min).to.be(moment(min).valueOf());
          expect(output.params.extended_bounds.max).to.be(moment(max).valueOf());
        });
      });

[rashidkpc]

min/max represent dates, that the only safe way to pass dates to Elasticsearch, without knowing the stored format, is as a long.

The reason I changed the original code to wrap the values in moment() was just in case something else passes in a long, I'll add a test for it as well per your suggestion.

[spalger]

LGTM