Ensure OperatorMatcher works with overriden method

Operator matchers break if comparing an object that overrides the `method` method.  Consider this example:

```ruby
require 'rspec/autorun'

class HttpModel
  def method
    :get
  end
end

describe HttpModel do
  it 'compares objects with overriden method' do
    myhttp = HttpModel.new
    myhttp.should == myhttp
  end
end
```

After upgrading to 2.14, this caused some of my tests to fail with the following error:

    1) HttpModel compares objects with overriden method
       Failure/Error: Unable to find matching line from backtrace
       ArgumentError:
         wrong number of arguments (1 for 0)
       # override-method.rb:4:in `method'
       # override-method.rb:12:in `block (2 levels) in <main>'

This pull request fixes this by always using `Object#method` instead of `instance.method`.