Fix golint failures on handlers/negotiation

DefaultEndpointRestrictions is only used in the module,
so this renames it to defaultEndpointRestrictions.

Kubernetes-commit: 302ec9859113f322a32ed03673865b32ca5a130a

pkg/endpoints/handlers/negotiation/errors.go

@@ -29,6 +29,7 @@ type errNotAcceptable struct {
 	accepted []string
 }
 
+// NewNotAcceptableError returns an error of NotAcceptable which contains specified string
 func NewNotAcceptableError(accepted []string) error {
 	return errNotAcceptable{accepted}
 }
@@ -51,6 +52,7 @@ type errUnsupportedMediaType struct {
 	accepted []string
 }
 
+// NewUnsupportedMediaTypeError returns an error of UnsupportedMediaType which contains specified string
 func NewUnsupportedMediaTypeError(accepted []string) error {
 	return errUnsupportedMediaType{accepted}
 }

pkg/endpoints/handlers/negotiation/negotiate.go

@@ -133,6 +133,8 @@ type EndpointRestrictions interface {
 	AllowsStreamSchema(schema string) bool
 }
 
+// DefaultEndpointRestrictions is the default EndpointRestrictions which allows
+// content-type negotiation to verify server support for specific options
 var DefaultEndpointRestrictions = emptyEndpointRestrictions{}
 
 type emptyEndpointRestrictions struct{}