add toJSON function to MockFirebaseUser

[joshAg]

fixes issue #121 by adding the function. I hacked together something for metadata and provider info, but it should at least not explode.

[dmurvihill]

Hi @joshAg, thanks for the contribution. This repo is no longer used (see #160) but I have opened a new PR on my fork, which will be used to publish a release in early December.

For starters, can you add test coverage for the new function? Cheers.