navigator.share() should throw a TypeError if all fields are omitted

[mgiuca]

The following call:

navigator.share({foo: 'xyz'})

Currently results in a successful share with no data.

It should reject with a TypeError.

Rationale: Empty shares are a bad user experience. They show up in the target application as empty strings which is confusing. There's the simple case of passing an empty dictionary to share, but we also have to consider the more plausible case of adding new fields in the future, and how sites that use the new API will interact with old UAs that don't support those new fields.

The current status quo is that if you share only new fields, you'll share nothing on an old UA.

We should offer feature detection for these new fields (see WebIDL #107) and then make it a TypeError if you fail to properly use the feature detection and only share new fields.

This is related to #47 which is about making navigator.share() equivalent to navigator.share({}). If we make this change, navigator.share() would remain a TypeError which makes me happy. I think navigator.share() being a successful share is nonsensical.