Fix NoMethodError when query hash has nil value

The error occurs when using the NON_RAILS_QUERY_STRING_NORMALIZER with a
hash whose key is a symbol and value is nil

lib/httparty/request.rb

@@ -14,7 +14,7 @@ class Request #:nodoc:
     NON_RAILS_QUERY_STRING_NORMALIZER = Proc.new do |query|
       Array(query).map do |key, value|
         if value.nil?
-          key
+          key.to_s
         elsif value.is_a?(Array)
           value.map {|v| "#{key}=#{URI.encode(v.to_s, Regexp.new("[^#{URI::PATTERN::UNRESERVED}]"))}"}
         else

spec/httparty/request_spec.rb

@@ -13,7 +13,7 @@
       URI.unescape(query_string).should == "foo=bar&foo=baz"
     end
 
-    context "when representing an array" do
+    context "when the query is an array" do
 
       it "doesn't include brackets" do
         query_string = normalizer[{:page => 1, :foo => %w(bar baz)}]
@@ -25,6 +25,13 @@
         query_string.should == "people=Bob%20Marley&people=Tim%20%26%20Jon"
       end
     end
+
+    context "when the query is a hash" do
+      it "correctly handles nil values" do
+        query_string = normalizer[{:page => 1, :per_page => nil}]
+        query_string.should == "page=1&per_page"
+      end
+    end
   end
 
   describe "initialization" do