fixed and optimize ek_layer, now ek_layer's fields are same with xml_layer

[whh4git]

now ek_layer's fields are same with xml_layer, keep the interface consistent.

>>> import pyshark
>>> pcap = pyshark.FileCapture("./device.pcapng", display_filter="http.response", use_ek=1)
>>> pcap.reset()
>>> pcap.next()["http"].field_names        
['content_length', 'content_length_header', 'content_type', 'time', 'server', 'response_line', 'response', 'response_code_desc', 'response_code', 'text', 'response_phrase', 'response_version', 'file_data', 'request_uri', 'request_in', 'date']
>>> pcap = pyshark.FileCapture("./device.pcapng", display_filter="http.response")           
>>> pcap.reset()
>>> pcap.next()["http"].field_names
['', 'response_version', 'response_code', 'response_code_desc', 'response_phrase', 'server', 'response_line', 'date', 'content_type', 'content_length_header', 'content_length', 'response', 'request_in', 'time', 'request_uri', 'file_data']