Validate that values for unrecognized query options are strings

[joshed-io]

If a non-string is passed as an query option value for a key we don't track (not :group_by or :filters, etc) we should validate that it is a string so it can be escaped later on.

Currently passing a non-string just throws a confusing exception:

>> Keen.count("votes", :max_age => 60)
NoMethodError: undefined method `gsub' for 60:Fixnum
        from /Users/dzello/.rbenv/versions/1.9.3-p448/lib/ruby/1.9.1/cgi/util.rb:7:in `escape'
        from /Users/dzello/.rbenv/versions/1.9.3-p448/lib/ruby/gems/1.9.1/gems/keen-0.8.8/lib/keen/client.rb:109:in `block in preprocess_params'
        from /Users/dzello/.rbenv/versions/1.9.3-p448/lib/ruby/gems/1.9.1/gems/keen-0.8.8/lib/keen/client.rb:109:in `each'
        from /Users/dzello/.rbenv/versions/1.9.3-p448/lib/ruby/gems/1.9.1/gems/keen-0.8.8/lib/keen/client.rb:109:in `map'
        from /Users/dzello/.rbenv/versions/1.9.3-p448/lib/ruby/gems/1.9.1/gems/keen-0.8.8/lib/keen/client.rb:109:in `preprocess_params'
        from /Users/dzello/.rbenv/versions/1.9.3-p448/lib/ruby/gems/1.9.1/gems/keen-0.8.8/lib/keen/client/querying_methods.rb:239:in `_query_url'
        from /Users/dzello/.rbenv/versions/1.9.3-p448/lib/ruby/gems/1.9.1/gems/keen-0.8.8/lib/keen/client/querying_methods.rb:223:in `query'
        from /Users/dzello/.rbenv/versions/1.9.3-p448/lib/ruby/gems/1.9.1/gems/keen-0.8.8/lib/keen/client/querying_methods.rb:19:in `count'
        from (irb):7
        from /Users/dzello/.rbenv/versions/1.9.3-p448/bin/irb:12:in `<main>'