Optimize hot path in escapeExpression

Avoid deoptimizations in v8 due to the duct type check on string instances.

Partial fix for #973

lib/handlebars/utils.js

@@ -60,21 +60,23 @@ export function indexOf(array, value) {
 
 
 export function escapeExpression(string) {
-  // don't escape SafeStrings, since they're already safe
-  if (string && string.toHTML) {
-    return string.toHTML();
-  } else if (string == null) {
-    return "";
-  } else if (!string) {
-    return string + '';
-  }
+  if (typeof string !== 'string') {
+    // don't escape SafeStrings, since they're already safe
+    if (string && string.toHTML) {
+      return string.toHTML();
+    } else if (string == null) {
+      return '';
+    } else if (!string) {
+      return string + '';
+    }
 
-  // Force a string conversion as this will be done by the append regardless and
-  // the regex test will do this transparently behind the scenes, causing issues if
-  // an object's to string has escaped characters in it.
-  string = "" + string;
+    // Force a string conversion as this will be done by the append regardless and
+    // the regex test will do this transparently behind the scenes, causing issues if
+    // an object's to string has escaped characters in it.
+    string = '' + string;
+  }
 
-  if(!possible.test(string)) { return string; }
+  if (!possible.test(string)) { return string; }
   return string.replace(badChars, escapeChar);
 }