Fix wrong argument exception when &:block passed to the expose method

[DmitryTsepelev]

I've faced this problem when Rubocop suggested to fix Style/SymbolProc problem in the following code:

expose :something do |object|
  object.any_method
end

After the fix the following code started to throw wrong argument number exception, because options are always passed to the block:

expose :something, &:any_method

I'm aware of the as: option which solves the case above, but probably this enhancement will help avoid confusion with Rubocop users.

[coveralls]

Coverage increased (+0.02%) to 92.76% when pulling 993b623 on DmitryTsepelev:block-shorthand-crash into 064ecfb on ruby-grape:master.

[DmitryTsepelev]

@LeFnord could you please review when you have a second? thanks :)

[LeFnord]

the rubocop warnings will be fixed in #287

[DmitryTsepelev]

@LeFnord Got it, thank you 👍