message: Add default to offset param of Python generated GetRootAs
diff: @@ -127,7 +127,7 @@ class PythonGenerator : public BaseGenerator {
  code += Indent + "@classmethod\n";
  code += Indent + "def GetRootAs";
  code += NormalizedName(struct_def);
  - code += "(cls, buf, offset):";
  + code += "(cls, buf, offset=0):";
  code += "\n";
  code += Indent + Indent;
  code += "n = flatbuffers.encode.Get";
  