Fix docs and behavior of WithContext (#499)

ctx.go

@@ -14,10 +14,15 @@ func init() {
 
 type ctxKey struct{}
 
-// WithContext returns a copy of ctx with l associated. If an instance of Logger
+// WithContext returns a copy of ctx with the receiver attached. The Logger
-// is already in the context, the context is not updated.
+// attached to the provided Context (if any) will not be effected.  If the
+// receiver's log level is Disabled it will only be attached to the returned
+// Context if the provided Context has a previously attached Logger. If the
+// provided Context has no attached Logger, a Disabled Logger will not be
+// attached.
 //
-// For instance, to add a field to an existing logger in the context, use this
+// Note: to modify the existing Logger attached to a Context (instead of
+// replacing it in a new Context), use UpdateContext with the following
 // notation:
 //
 //     ctx := r.Context()
@@ -25,13 +30,9 @@ type ctxKey struct{}
 //     l.UpdateContext(func(c Context) Context {
 //         return c.Str("bar", "baz")
 //     })
+//
 func (l Logger) WithContext(ctx context.Context) context.Context {
-	if lp, ok := ctx.Value(ctxKey{}).(*Logger); ok {
+	if _, ok := ctx.Value(ctxKey{}).(*Logger); !ok && l.level == Disabled {
-		if lp == &l {
-			// Do not store same logger.
-			return ctx
-		}
-	} else if l.level == Disabled {
 		// Do not store disabled logger.
 		return ctx
 	}