pymavlink: fixed assumption that type(x) returns just the type

it returns "<type 'bytearray'>" on some versions of python

pymavlink/mavutil.py

@@ -760,10 +760,9 @@ class mavserial(mavfile):
 
     def write(self, buf):
         try:
-            if type(buf) == 'str':
-                return self.port.write(buf)
-            if type(buf) == 'bytearray':
-                return self.port.write(str(buf)) 
+            if not isinstance(buf, str):
+                buf = str(buf)
+            return self.port.write(buf)
         except Exception:
             if not self.portdead:
                 print("Device %s is dead" % self.device)