Gmail's signature is not being detected

When you parse emails, you detect signature by `--` (dash-dash-space). But, Gmail for some odd reason, adds asterisks before and after signature delimiter, you should, probably, support this as well. In raw email it looks like this: `*-- *`.

Locate the signature-detection logic in this Ruby email parser and inspect how the `--` delimiter is recognized. Add coverage for Gmail's raw `*-- *` form and verify that parsing removes the signature while preserving the message body.

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