Corrected TwoWire::onRequestService(void) to correctly reset the write buffer before send frame. (arduino#22)

* Update Wire.cpp

Corrected TwoWire::onRequestService(void) to correctly reset the write buffer before send frame.
txBufferIndex and txBufferLength no longer control the buffer since TwoWire::write(uint8_t data) is modified for run with megaavr architecture.

Author: goodchip

libraries/Wire/src/Wire.cpp

@@ -341,10 +341,8 @@ void TwoWire::onRequestService(void)
 		return;
 	}
 
-	// reset tx buffer iterator vars
-	// !!! this will kill any pending pre-master sendTo() activity
-	txBufferIndex = 0;
-	txBufferLength = 0;
+	// reset slave write buffer iterator var
+	slave_bytesToWrite = 0;
 
 	// alert user program
 	user_onRequest();