Fix splitting for large frames. Fixes molnarg#207

I'm honestly not sure why this EVER worked - we were never limiting DATA
frames to anything less than the flow control window. This fixes that to
take into account the (default) max allowed payload size.

Tested and works fine for me with node 6.4.0

example/server.js

@@ -30,6 +30,17 @@ function onRequest(request, response) {
     fileStream.on('finish',response.end);
   }
 
+  // Example for testing large (boundary-sized) frames.
+  else if (request.url === "/largeframe") {
+    response.writeHead(200);
+    var body = 'a';
+    for (var i = 0; i < 14; i++) {
+      body += body;
+    }
+    body = body + 'a';
+    response.end(body);
+  }
+
   // Otherwise responding with 404.
   else {
     response.writeHead(404);

lib/protocol/flow.js

@@ -249,8 +249,9 @@ Flow.prototype._parentPush = function _parentPush(frame) {
 // did not push the whole frame to the output queue (but maybe it did push part of the frame).
 Flow.prototype._push = function _push(frame) {
   var data = frame && (frame.type === 'DATA') && frame.data;
+  var maxFrameLength = (this._window < 16384) ? this._window : 16384;
 
-  if (!data || (data.length <= this._window)) {
+  if (!data || (data.length <= maxFrameLength)) {
     return this._parentPush(frame);
   }
 
@@ -261,12 +262,12 @@ Flow.prototype._push = function _push(frame) {
   else {
     this._log.trace({ frame: frame, size: frame.data.length, forwardable: this._window },
                     'Splitting out forwardable part of a DATA frame.');
-    frame.data = data.slice(this._window);
+    frame.data = data.slice(maxFrameLength);
     this._parentPush({
       type: 'DATA',
       flags: {},
       stream: frame.stream,
-      data: data.slice(0, this._window)
+      data: data.slice(0, maxFrameLength)
     });
     return null;
   }