Allow whitespace in filter expression

Stencil/Variable.swift

@@ -6,7 +6,7 @@ class FilterExpression : Resolvable {
   let variable: Variable
 
   init(token: String, parser: TokenParser) throws {
-    let bits = token.characters.split("|").map(String.init)
+    let bits = token.characters.split("|").map({ String($0).trim(" ") })
     if bits.isEmpty {
       filters = []
       variable = Variable("")

StencilSpecs/FilterSpec.swift

@@ -30,6 +30,12 @@ describe("template filters") {
 
     try expect(try template.render(context, namespace: namespace)).toThrow(TemplateSyntaxError("No Repeat"))
   }
+
+  $0.it("allows whitespace in expression") {
+    let template = Template(templateString: "{{ name | uppercase }}")
+    let result = try template.render(Context(dictionary: ["name": "kyle"]))
+    try expect(result) == "KYLE"
+  }
 }