Fix File.get_as_text() to return the whole file

It was returning only from the cursor forward.

core/bind/core_bind.cpp

@@ -1566,7 +1566,12 @@ DVector<uint8_t> _File::get_buffer(int p_length) const{
 
 String _File::get_as_text() const {
 
+	ERR_FAIL_COND_V(!f, String());
+
 	String text;
+	size_t original_pos = f->get_pos();
+	f->seek(0);
+
 	String l = get_line();
 	while(!eof_reached()) {
 		text+=l+"\n";
@@ -1574,6 +1579,8 @@ String _File::get_as_text() const {
 	}
 	text+=l;
 
+	f->seek(original_pos);
+
 	return text;