Ruby oneof fields should have `has_foo?`, even for proto3 files

[haberman]

Currently Ruby suppresses has_foo? methods for proto3:

protobuf/ruby/ext/google/protobuf_c/message.c

Lines 246 to 254 in c578f59

	// Disallow oneof hazzers for proto3.
	// TODO(haberman): remove this test when we are enabling oneof hazzers for
	// proto3.
	if (*f && !upb_FieldDef_IsSubMessage(*f) &&
	upb_FieldDef_RealContainingOneof(*f) &&
	upb_MessageDef_Syntax(upb_FieldDef_ContainingType(*f)) !=
	kUpb_Syntax_Proto2) {
	return METHOD_UNKNOWN;
	}

As of 2020-09-15, it was decided that all oneof fields should have hazzers, even for proto3.

We should update Ruby to support has_foo? for oneof fields in Ruby.

(Note: in this case foo is the name of a field in a oneof, foo is not a oneof itself).

[haberman]

All we need to do is remove this if statement, and add a unit test to verify:

protobuf/ruby/ext/google/protobuf_c/message.c

Lines 245 to 253 in 7309e88

	// Disallow oneof hazzers for proto3.
	// TODO(haberman): remove this test when we are enabling oneof hazzers for
	// proto3.
	if (*f && !upb_FieldDef_IsSubMessage(*f) &&
	upb_FieldDef_RealContainingOneof(*f) &&
	upb_MessageDef_Syntax(upb_FieldDef_ContainingType(*f)) !=
	kUpb_Syntax_Proto2) {
	return METHOD_UNKNOWN;
	}