Write a PCAP header when no packets are provided

[guedou]

This PR fixes #1242

[p-l-]

@guedou this breaks the tests

[guedou]

@p-l- it looks ok now. I moved the unit test to a different location.

[karpierz]

Try to move it at the end of file.
I had the same problem with #1238 and still have very often with #1240 (despite the fact that I did not change anything in the tests)

[p-l-]

@guedou Can you rebase against current master?

[codecov-io]

Codecov Report

Merging #1253 into master will increase coverage by 1%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master    #1253     +/-   ##
=========================================
+ Coverage   83.98%   84.98%     +1%     
=========================================
  Files         159      160      +1     
  Lines       37546    38711   +1165     
=========================================
+ Hits        31532    32898   +1366     
+ Misses       6014     5813    -201

Impacted Files	Coverage Δ
scapy/utils.py	77.6% <100%> (+1.27%)	⬆️
scapy/fields.py	90.23% <0%> (-0.09%)	⬇️
scapy/arch/windows/__init__.py	79.01% <0%> (ø)
scapy/main.py	71.51% <0%> (+0.63%)	⬆️
scapy/modules/nmap.py	97.01% <0%> (+0.74%)	⬆️
scapy/layers/tls/handshake_sslv2.py	92.72% <0%> (+0.76%)	⬆️
scapy/supersocket.py	79.22% <0%> (+0.86%)	⬆️
scapy/layers/l2.py	82.8% <0%> (+1.14%)	⬆️
scapy/scapypipes.py	85.28% <0%> (+1.29%)	⬆️
... and 11 more

[guedou]

@p-l- done!