[Issue 252] Add type cast for default enum values in C#

When creating a “CreateXxx(...)” method for a “simple table” type,
enum-type fields with a non-zero default must have an explicit
cast for the respective argument default value, because in C#,
there is an implicit cast from int to an enum only for 0.

Also, added an example of such type into the example monster_test
type, so that we test this feature.

tests/MyGame/Example/TestSimpleTableWithEnum.go

@@ -0,0 +1,27 @@
+// automatically generated, do not modify
+
+package Example
+
+import (
+	flatbuffers "github.com/google/flatbuffers/go"
+)
+type TestSimpleTableWithEnum struct {
+	_tab flatbuffers.Table
+}
+
+func (rcv *TestSimpleTableWithEnum) Init(buf []byte, i flatbuffers.UOffsetT) {
+	rcv._tab.Bytes = buf
+	rcv._tab.Pos = i
+}
+
+func (rcv *TestSimpleTableWithEnum) Color() int8 {
+	o := flatbuffers.UOffsetT(rcv._tab.Offset(4))
+	if o != 0 {
+		return rcv._tab.GetInt8(o + rcv._tab.Pos)
+	}
+	return 2
+}
+
+func TestSimpleTableWithEnumStart(builder *flatbuffers.Builder) { builder.StartObject(1) }
+func TestSimpleTableWithEnumAddColor(builder *flatbuffers.Builder, color int8) { builder.PrependInt8Slot(0, color, 2) }
+func TestSimpleTableWithEnumEnd(builder *flatbuffers.Builder) flatbuffers.UOffsetT { return builder.EndObject() }