template: fix quote-handling with formatters

Fixes issue #1896.

R=golang-dev, dsymonds
CC=golang-dev
https://golang.org/cl/4539093

src/pkg/template/template.go

@@ -395,11 +395,12 @@ func words(buf []byte) []string {
 			} else {
 				i++
 			}
-		} else {
+		}
+		// Even with quotes, break on whitespace only.  This will
+		// work with e.g. {""|} and catch quoting mistakes properly.
 		for i < len(buf) && !white(buf[i]) {
 			i++
 		}
-		}
 		s = append(s, string(buf[start:i]))
 	}
 	return s

src/pkg/template/template_test.go

@@ -762,6 +762,10 @@ var formatterTests = []Test{
 		in:  `{"%.02f 0x%02X" 1.1 10|printf}`,
 		out: "1.10 0x0A",
 	},
+	{
+		in:  `{""|}{""||}{""|printf}`, // Issue #1896.
+		out: "",
+	},
 }
 
 func TestFormatters(t *testing.T) {