Add flag to prevent polling. Fix header type change.

- Introduce FLAG_NO_POLL which (optionally) prevents the network device
from responding to NETWORK_POLL requests
- Ensure the header.type is unmodified after fragmented writes per #64
- Considering this a bug because the behavior is inconsistent with a
standard write

RF24Network.cpp

@@ -234,11 +234,11 @@ uint8_t RF24Network::update(void)
 
 
 				if(header->type == NETWORK_POLL ){
-				    //Serial.println("Send poll");
-					header->to_node = header->from_node;
-					header->from_node = node_address;			
-					//delay((node_address%5)*3);
-					write(header->to_node,USER_TX_TO_PHYSICAL_ADDRESS);
+                    if( !(networkFlags & FLAG_NO_POLL) ){
+					  header->to_node = header->from_node;
+					  header->from_node = node_address;			
+					  write(header->to_node,USER_TX_TO_PHYSICAL_ADDRESS);
+                    }
 					continue;
 				}
 				uint8_t val = enqueue(header);
@@ -763,7 +763,7 @@ bool RF24Network::write(RF24NetworkHeader& header,const void* message, uint16_t
 	#endif
 
   }
-
+  header.type = type;
   #if !defined (DUAL_HEAD_RADIO)
   if(networkFlags & FLAG_FAST_FRAG){
 	radio.startListening();

RF24Network.h

@@ -180,6 +180,8 @@
  #define FLAG_BYPASS_HOLDS 2
 
  #define FLAG_FAST_FRAG 4
+
+ #define FLAG_NO_POLL 8
 
 class RF24;
 
@@ -755,6 +757,7 @@ class RF24Network
   * |FLAG_HOLD_INCOMING| 1(bit_1) | INTERNAL: Set automatically when a fragmented payload will exceed the available cache |
   * |FLAG_BYPASS_HOLDS| 2(bit_2) | EXTERNAL: Can be used to prevent holds from blocking. Note: Holds are disabled & re-enabled by RF24Mesh when renewing addresses. This will cause data loss if incoming data exceeds the available cache space|
   * |FLAG_FAST_FRAG| 4(bit_3) | INTERNAL: Replaces the fastFragTransfer variable, and allows for faster transfers between directly connected nodes. |
+  * |FLAG_NO_POLL| 8(bit_4) | EXTERNAL/USER: Disables NETWORK_POLL responses on a node-by-node basis. |  
   * 
   */
   uint8_t networkFlags;