MINOR: lua: add function which return true if the channel is full.

Add function which return true if the channel is full. It is
useful for triggering some process when the buffer is full.
diff --git a/doc/lua-api/_static/channel.fig b/doc/lua-api/_static/channel.fig
index 0838a37..8a6c0a1 100644
--- a/doc/lua-api/_static/channel.fig
+++ b/doc/lua-api/_static/channel.fig
@@ -8,12 +8,12 @@
 -2
 1200 2
 1 1 0 1 0 7 50 -1 -1 0.000 1 0.0000 4500 1620 1260 585 4500 1620 5760 2205
-2 3 0 1 0 7 50 -1 -1 0.000 0 0 -1 0 0 9
+2 3 0 1 0 7 50 -1 -1 0.000 0 0 -1 0 0 8
 	 1170 1350 1170 1890 2790 1890 2790 2070 3240 1620 2790 1170
-	 2790 1350 1170 1350 1170 1350
-2 3 0 1 0 7 50 -1 -1 0.000 0 0 -1 0 0 9
+	 2790 1350 1170 1350
+2 3 0 1 0 7 50 -1 -1 0.000 0 0 -1 0 0 8
 	 5760 1350 5760 1890 7380 1890 7380 2070 7830 1620 7380 1170
-	 7380 1350 5760 1350 5760 1350
+	 7380 1350 5760 1350
 2 1 1 1 0 7 50 -1 -1 1.000 0 0 -1 1 0 2
 	5 1 1.00 60.00 120.00
 	 6210 540 6210 1440
@@ -51,4 +51,5 @@
 4 0 0 50 -1 12 12 0.0000 4 150 720 6300 1110 send()\001
 4 0 0 50 -1 12 12 0.0000 4 165 1560 6255 2205 get_out_len()\001
 4 0 0 50 -1 16 12 0.0000 4 150 1230 6120 2520 read functions\001
-4 1 0 50 -1 16 12 0.0000 4 150 1650 4500 540 both side functions\001
+4 1 0 50 -1 16 12 0.0000 4 150 1650 4500 315 both side functions\001
+4 1 0 50 -1 12 12 0.0000 4 150 1080 4500 540 is_full()\001
diff --git a/doc/lua-api/_static/channel.png b/doc/lua-api/_static/channel.png
index 79eb6f7..e12a26e 100644
--- a/doc/lua-api/_static/channel.png
+++ b/doc/lua-api/_static/channel.png
Binary files differ
diff --git a/doc/lua-api/index.rst b/doc/lua-api/index.rst
index c42b2f3..77c2266 100644
--- a/doc/lua-api/index.rst
+++ b/doc/lua-api/index.rst
@@ -1070,6 +1070,11 @@
   :param class_channel channel: The manipulated Channel.
   :param integer int: The amount of data which will be forwarded.
 
+.. js:function:: Channel.is_full(channel)
+
+  This function returns true if the buffer channel is full.
+
+  :returns: a boolean
 
 .. _http_class: