fix query parametter with can't dup Object

Raise TypeError when query value is can't dup Object(etc Symbol, Fixnum).

```
 1) Webmock with Curb using #http for requests it should behave like Curb with WebMock when requests are stubbed based on query params should return stubbed response when stub declares query params as a hash with can't dup object
     Failure/Error: stub_request(:get, "www.example.com").with(:query => {"a" => 1}).to_return(:body => "abc")
     TypeError:
       can't dup Fixnum
     Shared Example Group: "Curb" called from ./spec/acceptance/curb/curb_spec.rb:274
     # ./lib/webmock/util/query_mapper.rb:172:in `dup'

```