Update #check_fetch_offense method

[connicov1]

Updated method #check_fetch_offense to verify method #fetch if it has a block and return a correct error message.

Related issue: #37

[DamirSvrtan]

Thank you for your contribution! It seems we are reversing this rule - has this rule been reversed in fast-ruby as well?

[connicov1]

Thank you for your contribution! It seems we are reversing this rule - has this rule been reversed in fast-ruby as well?

Hey, I’ve checked the fast-ruby and there are quite the same logic as I did here. Don’t know why they think a constant as argument is slower than a block, however, all the primitive data types are faster than a block in fast-ruby as well

[gagalago]

@DamirSvrtan I don't get you comment. This pull request revert this rule only for constant and primitive type, for the rest(method call, object...), this rule is still valid.