Avoid calling RR.Header more than once per RR

[tmthrgd]

Header is an interface method so there's non-zero overhead when calling it. This should be ever so slightly faster.

[codecov-io]

Codecov Report

Merging #891 into master will increase coverage by 0.07%.
The diff coverage is 96.42%.

@@            Coverage Diff             @@
##           master     #891      +/-   ##
==========================================
+ Coverage   56.37%   56.45%   +0.07%     
==========================================
  Files          41       41              
  Lines       10031    10031              
==========================================
+ Hits         5655     5663       +8     
+ Misses       3354     3348       -6     
+ Partials     1022     1020       -2

Impacted Files	Coverage Δ
sanitize.go	100% <100%> (ø)	⬆️
sig0.go	64.28% <100%> (-1.24%)	⬇️
update.go	71.92% <100%> (+2.11%)	⬆️
dnssec.go	58.74% <92.3%> (+0.46%)	⬆️
server.go	66.43% <0%> (+0.46%)	⬆️
msg.go	78.05% <0%> (+0.66%)	⬆️

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