Set max value for progress task properly

Also clamp the task value if it's greater than the max value.

Closes #163

src/Spectre.Console.Tests/Unit/ProgressTests.cs

@@ -59,7 +59,7 @@ namespace Spectre.Console.Tests.Unit
         }
 
         [Fact]
-        public Task Foo()
+        public Task Should_Reduce_Width_If_Needed()
         {
             // Given
             var console = new PlainConsole(width: 20);
@@ -87,5 +87,29 @@ namespace Spectre.Console.Tests.Unit
             // Then
             return Verifier.Verify(console.Output);
         }
+
+        [Fact]
+        public void Setting_Max_Value_Should_Set_The_MaxValue_And_Cap_Value()
+        {
+            // Given
+            var task = default(ProgressTask);
+            var console = new PlainConsole();
+            var progress = new Progress(console)
+                .Columns(new[] { new ProgressBarColumn() })
+                .AutoRefresh(false)
+                .AutoClear(false);
+
+            // When
+            progress.Start(ctx =>
+            {
+                task = ctx.AddTask("foo");
+                task.Increment(100);
+                task.MaxValue = 20;
+            });
+
+            // Then
+            task.MaxValue.ShouldBe(20);
+            task.Value.ShouldBe(20);
+        }
     }
 }

src/Spectre.Console/Progress/ProgressTask.cs

@@ -167,7 +167,7 @@ namespace Spectre.Console
 
                 if (maxValue != null)
                 {
-                    _maxValue += maxValue.Value;
+                    _maxValue = maxValue.Value;
                 }
 
                 if (increment != null)
@@ -175,6 +175,12 @@ namespace Spectre.Console
                     Value += increment.Value;
                 }
 
+                // Need to cap the max value?
+                if (Value > _maxValue)
+                {
+                    Value = _maxValue;
+                }
+
                 var timestamp = DateTime.Now;
                 var threshold = timestamp - TimeSpan.FromSeconds(30);