TrustedProxies: Add default IPv6 support and refactor

[Bisstocuz]

1. Tidy: Complete TrustedProxies feature #2887 didn't support IPv6 by default, now fixed it. (It return unexpected remote IP when nginx proxy pass to "localhost" address. #2966)
2. Move two functions with receiver Engine from context.go to gin.go.
3. Refactor for function validateHeader to decrease Codecov coverage misses.
4. Refactor for Context.RemoteIP():
   4.1. This is a public function, its usage should be friendly to users.
   4.2. RemoteAddr is almost impossible to fake. If I wanna get the remote address directly by RemoteIP(), it's unnecessary to know if trusted. The Trusted Proxies checking should be finished in ClientIP().
5. Improve checking logic in Engine.isUnsafeTrustedProxies().

This PR has passed checks on my side: Bisstocuz#3

[codecov]

Codecov Report

Merging #2967 (ddc5c55) into master (a06d546) will increase coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #2967      +/-   ##
==========================================
+ Coverage   98.57%   98.60%   +0.03%     
==========================================
  Files          41       41              
  Lines        3085     3084       -1     
==========================================
  Hits         3041     3041              
+ Misses         31       30       -1     
  Partials       13       13              

Flag	Coverage Δ
go-1.13	∅ <ø> (∅)
go-1.14	∅ <ø> (∅)
go-1.15	∅ <ø> (∅)
go-1.16	98.60% <100.00%> (+0.03%)	⬆️
go-1.17	98.50% <100.00%> (+0.03%)	⬆️
macos-latest	98.60% <100.00%> (+0.03%)	⬆️
nomsgpack	98.58% <100.00%> (+0.03%)	⬆️
ubuntu-latest	98.60% <100.00%> (+0.03%)	⬆️

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Impacted Files	Coverage Δ
context.go	97.89% <100.00%> (+0.08%)	⬆️
gin.go	99.17% <100.00%> (+0.05%)	⬆️

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[Bisstocuz]

Linked issue #2966 by @beanbest

Code review plz @thinkerou

[Bisstocuz]

@appleboy code review plz

[thinkerou]

lgtm