Fix hiding the 'say' bubble WRT number formatting

When setting bubble text, numbers are rounded to two decimal places if
necessary. The check before hiding the bubble that makes sure the
current bubble text matches the current value -- probably intended to
make sure it's "our" bubble -- now uses the same text formatting code to
avoid false negatives.
In my mind the balance is this:
- Arguments for: consistency (major), might be useful (minor)
- Arguments against: currently not necessary (minor)

If I were adding the 'source' variable just for this case then I would totally agree that it's a bad idea to make a change just because it might be useful some day. However, since some cases already need this variable, it makes me uncomfortable to intentionally set it to a "wrong" value (null) just because we don't currently need it to be correct. I feel like it's a least-astonishment violation.