[compress] Fix compressible-status-code config

[masaori335]

The compressible-status-code config of compress plugin doesn't work as expected. Because the set of status code has default value but it's not cleared before insert. So setting like compressible-status-code 200,304 doesn't update the set.

trafficserver/plugins/compress/configuration.h

Lines 155 to 156 in 07888c0

	std::set<TSHttpStatus> compressible_status_codes_ = {TS_HTTP_STATUS_OK, TS_HTTP_STATUS_PARTIAL_CONTENT,
	TS_HTTP_STATUS_NOT_MODIFIED};