GenericDocument::ParseStream: make SourceEncoding optional

The ParseStream() function current requires explicitly passing
the SourceEncoding parameter as explicit template argument
while the other Parse*() functions provide overloads to omit
this parameter and use the document's own encoding by default.

This patch adds the corresponding overload for ParseStream(),
enabling the simple usage again:

  rapidjson::FileStream is(fp);
  rapidjson::Document   d;
  d.ParseStream<0>(is);