Add toString method to process.js

[cprecioso]

In Node.js:

process + "" // = "[object process]"

This patch just brings that behaviour over. I've seen this check in the wild (namely here).

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I've also made a PR at defunctzombie/node-process#87. I don't know if this is appropriate for the mock object here.

[jsf-clabot]

All committers have signed the CLA.

[sokra]

The mock is intentional minimal and doesn't need this function