#80: GetAddrHeader and SetGenHeader

[wneessen]

This PR introduces two major changes:

• SetHeader and SetHeaderPreformatted have been deprecated in favour of SetGenHeader and SetGenHeaderPreformatted As pointed out in #80 the naming was pretty confusing, given that we already have SetAddrHeader. With the new naming convention it should be more clear. For compatibility reasons the old methods have been kept for now but in reality they are just aliases to the new methods
• GetAddrHeader and GetAddrHeaderString have been introduced As requested in #80 analogous to GetGenHeader we also need a similar method for the address headers. Since address headers are *mail.Address pointer, we've also added a *String method that will extract the address string and return a string slice instead Additionally we're introducing methods for the actual address headers: GetTo, GetFrom, GetCc and GetBcc (with a *String counterpart as well). This way the user has full flexibility. Either they use the more "low-level" GetAddrHeader method or the higher level methods for the corresponding address type

[codecov-commenter]

Codecov Report

Merging #83 (17b9d2c) into main (1ff87db) will decrease coverage by 0.62%. The diff coverage is 93.02%.

@@            Coverage Diff             @@
##             main      #83      +/-   ##
==========================================
- Coverage   83.85%   83.22%   -0.63%     
==========================================
  Files          13       13              
  Lines        1381     1425      +44     
==========================================
+ Hits         1158     1186      +28     
- Misses        153      165      +12     
- Partials       70       74       +4     

Impacted Files	Coverage Δ
msg.go	86.49% <93.02%> (-0.04%)	:arrow_down:
msgwriter.go	86.08% <0.00%> (-4.57%)	:arrow_down:

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