Hash does not respond to delete!

In `lib/github_api/core_ext/hash.rb` lines 13-17, `except!` method does not work as is using a non-existent `delete!` method:

```ruby
def except!(*keys)
  copy = self.dup
  keys.each { |key| copy.delete!(key) }
  copy
end
``` 

There's a nasty re-monkeypatch in `spec_helper.rb` which is masking a bug in the real code:

- `spec_helper.rb` snippet:

```ruby
class Hash
  def except(*keys)
    cpy = self.dup
    keys.each { |key| cpy.delete(key) }
    cpy
  end
end
```

After removing this, the test suite explodes:

```
Finished in 6.09 seconds
1743 examples, 128 failures
```

So `Hash` does not respond to `delete!`, so implement `reject!` using `delete` instead of `delete!`.