Fix (Ether() / ARP()).show()

[p-l-]

Bug introduced in 435918e

Test added.

[codecov-io]

Codecov Report

Merging #1774 into master will increase coverage by 0.03%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master   #1774      +/-   ##
=========================================
+ Coverage   85.57%   85.6%   +0.03%     
=========================================
  Files         181     181              
  Lines       42206   42209       +3     
=========================================
+ Hits        36117   36133      +16     
+ Misses       6089    6076      -13

Impacted Files	Coverage Δ
scapy/layers/l2.py	83.66% <100%> (+0.37%)	⬆️
scapy/layers/ntp.py	91.77% <0%> (+0.26%)	⬆️
scapy/asn1/ber.py	82.38% <0%> (+0.28%)	⬆️
scapy/layers/tls/basefields.py	80.53% <0%> (+0.67%)	⬆️
scapy/layers/tls/handshake_sslv2.py	92.77% <0%> (+0.76%)	⬆️
scapy/layers/tls/record.py	92.1% <0%> (+0.87%)	⬆️
scapy/arch/bpf/core.py	82.1% <0%> (+1.05%)	⬆️
scapy/layers/tls/record_sslv2.py	88.7% <0%> (+1.69%)	⬆️