fix: detect expectations in chains with underscores

[ghusse]

Fixes expectation detection in call chains when one member contains an underscore.

For instance

it('should pass', () => {
  any_chain.to.expect(true)
})

Does not currently pass, whereas the following does pass

it('should pass', () => {
  anyChain.to.expect(true)
})

[veritem]

thanks