Allow query string parameters without values

Addresses #624

src/Microsoft.AspNetCore.WebUtilities/QueryHelpers.cs

@@ -140,7 +140,6 @@ namespace Microsoft.AspNetCore.WebUtilities
                 scanIndex = 1;
             }
 
-
             int textLength = queryString.Length;
             int equalIndex = queryString.IndexOf('=');
             if (equalIndex == -1)
@@ -171,6 +170,13 @@ namespace Microsoft.AspNetCore.WebUtilities
                         equalIndex = textLength;
                     }
                 }
+                else
+                {
+                    if (delimiterIndex > scanIndex)
+                    {
+                        accumulator.Append(queryString.Substring(scanIndex, delimiterIndex - scanIndex), string.Empty);
+                    }
+                }
                 scanIndex = delimiterIndex + 1;
             }
 

test/Microsoft.AspNetCore.Http.Tests/Features/QueryFeatureTests.cs

@@ -24,5 +24,44 @@ namespace Microsoft.AspNetCore.Http.Features
             // Assert
             Assert.Equal("bar", queryCollection["foo"]);
         }
+
+        [Theory]
+        [InlineData("?q", "q")]
+        [InlineData("?q&", "q")]
+        [InlineData("?q1=abc&q2", "q2")]
+        [InlineData("?q=", "q")]
+        [InlineData("?q=&", "q")]
+        public void KeyWithoutValuesAddedToQueryCollection(string queryString, string emptyParam)
+        {
+            var features = new FeatureCollection();
+            var request = new HttpRequestFeature();
+            request.QueryString = queryString;
+            features[typeof(IHttpRequestFeature)] = request;
+
+            var provider = new QueryFeature(features);
+
+            var queryCollection = provider.Query;
+
+            Assert.True(queryCollection.Keys.Contains(emptyParam));
+            Assert.Equal(string.Empty, queryCollection[emptyParam]);
+        }
+
+        [Theory]
+        [InlineData("?&&")]
+        [InlineData("?&")]
+        [InlineData("&&")]
+        public void EmptyKeysNotAddedToQueryCollection(string queryString)
+        {
+            var features = new FeatureCollection();
+            var request = new HttpRequestFeature();
+            request.QueryString = queryString;
+            features[typeof(IHttpRequestFeature)] = request;
+
+            var provider = new QueryFeature(features);
+
+            var queryCollection = provider.Query;
+
+            Assert.Equal(0, queryCollection.Count);
+        }
     }
 }