Fix URL building for when the base URL already has query parameters, and add test.

lib/stripe.rb

@@ -124,7 +124,7 @@ module Stripe
       # Make params into GET parameters
       if params && params.count > 0
         query_string = Util.flatten_params(params).collect{|key, value| "#{key}=#{Util.url_encode(value)}"}.join('&')
-        url += "?#{query_string}"
+        url += "#{URI.parse(url).query ? '&' : '?'}#{query_string}"
       end
       payload = nil
     else

test/test_helper.rb

@@ -189,6 +189,14 @@ def test_paid_invoice
     })
 end
 
+def test_invoice_array
+  {
+    :data => [test_invoice],
+    :object => 'list',
+    :url => '/v1/invoices/upcoming?customer=test_customer'
+  }
+end
+
 def test_invalid_api_key_error
   {
     "error" => {

test/test_stripe.rb

@@ -120,6 +120,15 @@ class TestStripeRuby < Test::Unit::TestCase
         assert charges.kind_of? Array
       end
 
+      should "construct URL properly with base query parameters" do
+        response = test_response(test_invoice_array)
+        @mock.expects(:get).with("#{Stripe.api_base}/v1/invoices/upcoming?customer=test_customer", nil, nil).returns(response)
+        invoices = Stripe::Invoice.upcoming(:customer => 'test_customer')
+
+        @mock.expects(:get).with("#{Stripe.api_base}/v1/invoices/upcoming?customer=test_customer&paid=true", nil, nil).returns(response)
+        invoices.all(:paid => true)
+      end
+
       should "a 400 should give an InvalidRequestError with http status, body, and JSON body" do
         response = test_response(test_missing_id_error, 400)
         @mock.expects(:get).once.raises(RestClient::ExceptionWithResponse.new(response, 404))