String indexer should return nil (#540)

Fixes #539

Author: sebastienros

Fluid.Tests/TemplateTests.cs

@@ -364,14 +364,14 @@ public async Task ShouldOnlyAllowInheritedMember()
         }
 
         [Fact]
-        public async Task ShouldEvaluateStringIndex()
+        public async Task StringIndexerReturnsNil()
         {
-            _parser.TryParse("{{ x[1] }}", out var template, out var error);
+            _parser.TryParse("{% if x[0] == blank %}true{% else %}false{% endif %}", out var template, out var error);
             var context = new TemplateContext();
             context.SetValue("x", "abc");
 
             var result = await template.RenderAsync(context);
-            Assert.Equal("b", result);
+            Assert.Equal("true", result);
         }
 
         [Fact]

Fluid/Values/StringValue.cs

@@ -103,13 +103,7 @@ public override bool Equals(FluidValue other)
 
         protected override FluidValue GetIndex(FluidValue index, TemplateContext context)
         {
-            var i = (int) index.ToNumberValue();
-
-            if (i < _value.Length)
-            {
-                return Create(_value[i]);
-            }
-
+            // Indexer on string values should return nil.
             return NilValue.Instance;
         }