[protocolbuffers\message] clear method should be clear 1 field

current `clear` method renamed `clearAll($clear_unknown_fields = true)` method.

ProtocolBuffers\Message::clear($name);
[protocolbuffers\descriptor_builder] add testcase
[protocolbuffers\descriptor_builder] change setExtensionRange API to addExtensionRange.

Extension feature is able to keep multiple extension range values. god.