Fixed a syntax mistake regarding blocks in specs.

The block wasn't passed to `raise_error`, due to missing parenthesis
around it and the block.
Adding them would've looked a bit awkward, so I put the second
expectation in a lambda and passed it as a block to `raise_error`.
Merge pull request #2423 from razielgn/fixed-syntax-confusion-in-arguments-spec

Fixed a syntax confusion regarding blocks in specs.