bug in ServiceTracker.send

[MatrixFrog]

Reported this internally a while ago. I don't remember the full context so just copying and pasting what I said before:

When we clone at https://github.com/GoogleChrome/chrome-platform-analytics/blob/0265be122948f2ee7b8e323b80a0ce0f3c9d2c87/src/internal/service_tracker.js#L92 we get all the entries from this.params_, including those that have null/undefined for the value. That's why testSend_OmitsNullAndUndefinedValues should be failing. (The only reason it isn't failing is because there's a bug in ParameterMap.equals.)

We could fix this by going through the clone and removing null/undefined entries, but if we don't want to send them, why don't we just change ParameterMap.set to no-op if the value is null/undefined?