Fix bad bug with the "fast child" path: log.child({...}, true).

Timing impact. Before (numbers from my Mac):
    $ node tools/timechild.js
    Time `log.child`:
     - adding no fields:  0.01307ms per iteration
     - adding one field:  0.01336ms per iteration
     - adding two fields: 0.0134ms per iteration
     - adding serializer and one field: 0.01839ms per iteration
     - adding a (stderr) stream and one field: 0.01905ms per iteration
     - [fast] adding no fields:  0.000821ms per iteration
     - [fast] adding one field:  0.001067ms per iteration
     - [fast] adding two fields: 0.00122ms per iteration

After:
    $ node tools/timechild.js
    Time `log.child`:
     - adding no fields:  0.01243ms per iteration
     - adding one field:  0.01345ms per iteration
     - adding two fields: 0.01343ms per iteration
     - adding serializer and one field: 0.01671ms per iteration
     - adding a (stderr) stream and one field: 0.01915ms per iteration
     - [fast] adding no fields:  0.001742ms per iteration
     - [fast] adding one field:  0.00193ms per iteration
     - [fast] adding two fields: 0.002388ms per iteration

I.e., not great, but still 5x faster for the "fast child" path.

CHANGES.md

@@ -2,7 +2,12 @@
 
 ## bunyan 0.6.8 (not yet released)
 
-(nothing yet)
+- Fix bad bug in `log.child({...}, true);` where the added child fields **would
+  be added to the parent's fields**. This bug only existed for the "fast child"
+  path (that second `true` argument). A side-effect of fixing this is that
+  the "fast child" path is only 5 times as fast as the regular `log.child`,
+  instead of 10 times faster.
+
 
 ## bunyan 0.6.7
 

lib/bunyan.js

@@ -239,19 +239,24 @@ function Logger(options, _childOptions, _childSimple) {
 
   // Fast path for simple child creation.
   if (parent && _childSimple) {
-    // Single to stream close handling that this child owns none of its
-    // streams.
+    // `_isSimpleChild` is a signal to stream close handling that this child
+    // owns none of its streams.
     this._isSimpleChild = true;
 
     this._level = parent._level;
     this.streams = parent.streams;
     this.serializers = parent.serializers;
     this.src = parent.src;
-    this.fields = parent.fields;
+    var fields = this.fields = {};
+    var parentFieldNames = Object.keys(parent.fields);
+    for (var i = 0; i < parentFieldNames.length; i++) {
+      var name = parentFieldNames[i];
+      fields[name] = parent.fields[name];
+    }
     var names = Object.keys(options);
     for (var i = 0; i < names.length; i++) {
       var name = names[i];
-      this.fields[name] = options[name];
+      fields[name] = options[name];
     }
     return;
   }