THRIFT-4650: fix required fields incorrectly being marked as set

This closes #1610.

Client: go
diff --git a/lib/go/test/RequiredFieldTest.thrift b/lib/go/test/RequiredFieldTest.thrift
new file mode 100644
index 0000000..4a2dcae
--- /dev/null
+++ b/lib/go/test/RequiredFieldTest.thrift
@@ -0,0 +1,7 @@
+struct RequiredField {
+  1: required string name
+}
+
+struct OtherThing {
+  1: required i16 value
+}