src: make Nghttp2Stream a class

Currently there is a compiler warning which only showed up after
applying the fix for nodejs#85:
In file included from ../src/node-http2-core.cc:1:
../src/node-http2-core.h:242:1: warning: 'Nghttp2Stream' defined as a
struct here but
      previously declared as a class [-Wmismatched-tags]
struct Nghttp2Stream : public
std::enable_shared_from_this<Nghttp2Stream> {
^
../src/node-http2-core.h:19:1: note: did you mean struct here?
class Nghttp2Stream;
^~~~~
struct
In file included from ../src/node-http2-core.cc:1:

This commit attempts to fix this warning.

PR-URL: nodejs#86
Ref: nodejs#85
Reviewed-By: Colin Ihrig <[email protected]>
Reviewed-By: Anna Henningsen <[email protected]>
Reviewed-By: James M Snell <[email protected]>

Author: danbev

src/node_http2_core.h

@@ -239,7 +239,8 @@ class Nghttp2Session {
   friend class Nghttp2Stream;
 };
 
-struct Nghttp2Stream : public std::enable_shared_from_this<Nghttp2Stream> {
+class Nghttp2Stream : public std::enable_shared_from_this<Nghttp2Stream> { 
+ public:
   inline ~Nghttp2Stream();
 
   inline int Write(