Allow Received-SPF header back.

[jcbf]

By request in #31 .
Also spamassassin check if this header is present and reuses the evaluation.

[codecov-io]

Codecov Report

Merging #32 into master will decrease coverage by 1.66%.
The diff coverage is 7.14%.

@@            Coverage Diff             @@
##           master      #32      +/-   ##
==========================================
- Coverage   63.47%   61.81%   -1.67%     
==========================================
  Files           1        1              
  Lines         460      474      +14     
  Branches      198      204       +6     
==========================================
+ Hits          292      293       +1     
- Misses         84       95      +11     
- Partials       84       86       +2

Impacted Files	Coverage Δ
smf-spf.c	61.81% <7.14%> (-1.67%)	⬇️

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[coveralls]

Coverage decreased (-1.8%) to 80.335% when pulling 7494cec on jcbf/received_spf_header into 451d113 on master.