Fix issue where empty query key causes a `TypeError`

As seen in the real world (tm), sometimes queries have values but no keys which
causes webmock to blow up with a `TypeError`:

```
lib/webmock/util/query_mapper.rb:187:in `dup': can't dup NilClass (TypeError)
  from ./gems/webmock-1.15.2/lib/webmock/util/query_mapper.rb:187:in `block in values_to_query'
  from ./gems/webmock-1.15.2/lib/webmock/util/query_mapper.rb:185:in `each'
  from ./gems/webmock-1.15.2/lib/webmock/util/query_mapper.rb:185:in `values_to_query'
  from ./gems/webmock-1.15.2/lib/webmock/util/uri.rb:18:in `block in <class:URI>'
  from ./gems/webmock-1.15.2/lib/webmock/util/uri.rb:33:in `yield'
  from ./gems/webmock-1.15.2/lib/webmock/util/uri.rb:33:in `normalize_uri'
  from ./gems/webmock-1.15.2/lib/webmock/request_signature.rb:10:in `initialize'
  from ./gems/webmock-1.15.2/lib/webmock/http_lib_adapters/net_http.rb:290:in `new'
  from ./gems/webmock-1.15.2/lib/webmock/http_lib_adapters/net_http.rb:290:in `request_signature_from_request'
  from ./gems/webmock-1.15.2/lib/webmock/http_lib_adapters/net_http.rb:75:in `request'
  from ./gems/net-http-persistent-2.9/lib/net/http/persistent.rb:986:in `request'
  from ./bundler/gems/mechanize-3c0a3714926e/lib/mechanize/http/agent.rb:259:in `fetch'
  from ./bundler/gems/mechanize-3c0a3714926e/lib/mechanize.rb:440:in `get'
```

This resolves that by casting `nil` to a string as with `Symbol`.