added changes for fix and fixed tests

Author: Faraz Ahmed

src/JsonApiDotNetCore/Services/QueryComposer.cs

@@ -31,7 +31,7 @@ public string Compose(IJsonApiContext jsonApiContext)
         private string ComposeSingleFilter(FilterQuery query)
         {
             var result = "&filter";
-            result += QueryConstants.OPEN_BRACKET + query.Attribute + QueryConstants.CLOSE_BRACKET + query.Operation + query.Value;
+            result += QueryConstants.OPEN_BRACKET + query.Attribute + QueryConstants.CLOSE_BRACKET + "=" + query.Operation + ":" + query.Value;
             return result;
         }
     }

test/UnitTests/Services/QueryComposerTests.cs

@@ -17,10 +17,10 @@ public QueryComposerTests()
         }
 
         [Fact]
-        public void Can_Compose_FilterStringForUrl()
+        public void Can_ComposeEqual_FilterStringForUrl()
         {
             // arrange
-            var filter = new FilterQuery("attribute", "value", "=");
+            var filter = new FilterQuery("attribute", "value", "eq");
             var querySet = new QuerySet();
             List<FilterQuery> filters = new List<FilterQuery>();
             filters.Add(filter);
@@ -34,7 +34,28 @@ public void Can_Compose_FilterStringForUrl()
             // act
             var filterString = queryComposer.Compose(_jsonApiContext.Object);
             // assert
-            Assert.Equal("&filter[attribute]=value", filterString);
+            Assert.Equal("&filter[attribute]=eq:value", filterString);
+        }
+
+        [Fact]
+        public void Can_ComposeLessThan_FilterStringForUrl()
+        {
+            // arrange
+            var filter = new FilterQuery("attribute", "value", "le");
+            var querySet = new QuerySet();
+            List<FilterQuery> filters = new List<FilterQuery>();
+            filters.Add(filter);
+            querySet.Filters = filters;
+
+            _jsonApiContext
+                .Setup(m => m.QuerySet)
+                .Returns(querySet);
+
+            var queryComposer = new QueryComposer();
+            // act
+            var filterString = queryComposer.Compose(_jsonApiContext.Object);
+            // assert
+            Assert.Equal("&filter[attribute]=le:value", filterString);
         }
 
         [Fact]
@@ -54,4 +75,4 @@ public void NoFilter_Compose_EmptyStringReturned()
             Assert.Equal("", filterString); 
         }
     }
-}
+}