issue #9 - stream fails while trying to handle sparse indexes

Stream fails trying to call `round` on TrueClass, for sparse: true in the key of an index. This PR checks that the value of the hash is numeric before trying to round it. 

If the key of the index contains: `"key" : { "some.document.field" : 1, "sparse" : true }`
Then stream will send the following to the outlet in the trigger (`index_key.to_s`): `[["some.document.field", 1], ["sparse", true]]`