Fix Hashie::Mash#respond_to? with a suffixed key

This simple change just modifies the `Hashie::Mash#respond_to?` method so that it complies with the `#method_missing` behaviour.

Before the fix:

``` ruby
Hashie::Mash.new(abc: true).respond_to?(:abc?) # => false
Hashie::Mash.new(abc: true).respond_to?(:abc=) # => false
```

After the fix:

``` ruby
Hashie::Mash.new(abc: true).respond_to?(:abc?) # => true
```