goprotobuf: Implement SetDefaults function.

R=r
CC=golang-dev
http://codereview.appspot.com/4972046
diff --git a/proto/equal.go b/proto/equal.go
index bbcca82..3a12a7f 100644
--- a/proto/equal.go
+++ b/proto/equal.go
@@ -51,6 +51,8 @@
     corresponding fields are equal, unknown field sets
     are equal, and extensions sets are equal.
   - Two set scalar fields are equal iff their values are equal.
+    If the fields are of a floating-point type, remember that
+    NaN != x for all x, including NaN.
   - Two repeated fields are equal iff their lengths are the same,
     and their corresponding elements are equal.
   - Two unset fields are equal.