Update to last common bleve (#3986)

vendor/github.com/philhofer/fwd/writer_unsafe.go

@@ -0,0 +1,18 @@
+// +build !appengine
+
+package fwd
+
+import (
+	"reflect"
+	"unsafe"
+)
+
+// unsafe cast string as []byte
+func unsafestr(b string) []byte {
+	l := len(b)
+	return *(*[]byte)(unsafe.Pointer(&reflect.SliceHeader{
+		Len:  l,
+		Cap:  l,
+		Data: (*reflect.StringHeader)(unsafe.Pointer(&b)).Data,
+	}))
+}