Add UnsetHeader to go library

Client: go

We already have SetHeader and GetHeader helper functions in the go
library to deal with THeader injected into the context object. But we
didn't provide a way to unset/delete a key from the context object.

This will be useful with the TSimpleServer.SetForwardHeaders API. In the
scenario that a thrift server want to auto forward certain headers to
other upstream thrift servers as the fallback, but during the handling
of the request might decide to remove some of the auto forward headers.
This is also achievable through mutate the write header list, but since
that's a list, finding one key from the list and remove it is much more
hassle.

lib/go/thrift/header_context.go

@@ -44,6 +44,15 @@ func SetHeader(ctx context.Context, key, value string) context.Context {
 	)
 }
 
+// UnsetHeader unsets a previously set header in the context.
+func UnsetHeader(ctx context.Context, key string) context.Context {
+	return context.WithValue(
+		ctx,
+		headerKey(key),
+		nil,
+	)
+}
+
 // GetHeader returns a value of the given header from the context.
 func GetHeader(ctx context.Context, key string) (value string, ok bool) {
 	if v := ctx.Value(headerKey(key)); v != nil {

lib/go/thrift/header_context_test.go

@@ -25,7 +25,7 @@ import (
 	"testing"
 )
 
-func TestSetGetHeader(t *testing.T) {
+func TestSetGetUnsetHeader(t *testing.T) {
 	const (
 		key   = "foo"
 		value = "bar"
@@ -68,6 +68,17 @@ func TestSetGetHeader(t *testing.T) {
 			}
 		},
 	)
+
+	t.Run(
+		"Unset",
+		func(t *testing.T) {
+			ctx := UnsetHeader(ctx, key)
+
+			if _, ok := GetHeader(ctx, key); ok {
+				t.Errorf("GetHeader returned ok on unset key %q", key)
+			}
+		},
+	)
 }
 
 func TestReadKeyList(t *testing.T) {