Fix for r40067

Fix these failed examples:

```
1)
Array#pack with format 'Q' raises ArgumentError when the format modifier is '_' FAILED
Expected ArgumentError but no exception was raised ( was returned)
/Users/mrkn/chkbuild/tmp/build/20130402T120332Z/rubyspec/core/array/pack/shared/basic.rb:59:in `block (2 levels) in <top (required)>'
/Users/mrkn/chkbuild/tmp/build/20130402T120332Z/rubyspec/core/array/pack/q_spec.rb:7:in `<top (required)>'

2)
Array#pack with format 'Q' raises ArgumentError when the format modifier is '!' FAILED
Expected ArgumentError but no exception was raised ( was returned)
/Users/mrkn/chkbuild/tmp/build/20130402T120332Z/rubyspec/core/array/pack/shared/basic.rb:63:in `block (2 levels) in <top (required)>'
/Users/mrkn/chkbuild/tmp/build/20130402T120332Z/rubyspec/core/array/pack/q_spec.rb:7:in `<top (required)>'

3)
Array#pack with format 'q' raises ArgumentError when the format modifier is '_' FAILED
Expected ArgumentError but no exception was raised ( was returned)
/Users/mrkn/chkbuild/tmp/build/20130402T120332Z/rubyspec/core/array/pack/shared/basic.rb:59:in `block (2 levels) in <top (required)>'
/Users/mrkn/chkbuild/tmp/build/20130402T120332Z/rubyspec/core/array/pack/q_spec.rb:16:in `<top (required)>'

4)
Array#pack with format 'q' raises ArgumentError when the format modifier is '!' FAILED
Expected ArgumentError but no exception was raised ( was returned)
/Users/mrkn/chkbuild/tmp/build/20130402T120332Z/rubyspec/core/array/pack/shared/basic.rb:63:in `block (2 levels) in <top (required)>'
/Users/mrkn/chkbuild/tmp/build/20130402T120332Z/rubyspec/core/array/pack/q_spec.rb:16:in `<top (required)>'

5)
String#unpack with format 'Q' raises an ArgumentError when the format modifier is '_' FAILED
Expected ArgumentError
but no exception was raised ([7523094288207667809] was returned)
/Users/mrkn/chkbuild/tmp/build/20130402T120332Z/rubyspec/core/string/unpack/shared/basic.rb:23:in `block (2 levels) in <top (required)>'
/Users/mrkn/chkbuild/tmp/build/20130402T120332Z/rubyspec/core/string/unpack/q_spec.rb:32:in `<top (required)>'

6)
String#unpack with format 'Q' raises an ArgumentError when the format modifier is '!' FAILED
Expected ArgumentError
but no exception was raised ([7523094288207667809] was returned)
/Users/mrkn/chkbuild/tmp/build/20130402T120332Z/rubyspec/core/string/unpack/shared/basic.rb:27:in `block (2 levels) in <top (required)>'
/Users/mrkn/chkbuild/tmp/build/20130402T120332Z/rubyspec/core/string/unpack/q_spec.rb:32:in `<top (required)>'

7)
String#unpack with format 'q' raises an ArgumentError when the format modifier is '_' FAILED
Expected ArgumentError
but no exception was raised ([7523094288207667809] was returned)
/Users/mrkn/chkbuild/tmp/build/20130402T120332Z/rubyspec/core/string/unpack/shared/basic.rb:23:in `block (2 levels) in <top (required)>'
/Users/mrkn/chkbuild/tmp/build/20130402T120332Z/rubyspec/core/string/unpack/q_spec.rb:37:in `<top (required)>'

8)
String#unpack with format 'q' raises an ArgumentError when the format modifier is '!' FAILED
Expected ArgumentError
but no exception was raised ([7523094288207667809] was returned)
/Users/mrkn/chkbuild/tmp/build/20130402T120332Z/rubyspec/core/string/unpack/shared/basic.rb:27:in `block (2 levels) in <top (required)>'
/Users/mrkn/chkbuild/tmp/build/20130402T120332Z/rubyspec/core/string/unpack/q_spec.rb:37:in `<top (required)>'
```