Improve Contact header parsing

[jlaine]

• accept all tokens in unquoted name
• don't put whitespace into name
• correctly parse params when there is a space before params

Also:

• add tests for transforming the Contact back to a header string
• remove dead 'from_repr' code

[codecov-io]

Codecov Report

Merging #64 into master will increase coverage by 0.49%. The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #64      +/-   ##
==========================================
+ Coverage   70.72%   71.22%   +0.49%     
==========================================
  Files          15       15              
  Lines        1455     1449       -6     
  Branches      260      259       -1     
==========================================
+ Hits         1029     1032       +3     
+ Misses        315      309       -6     
+ Partials      111      108       -3

Impacted Files	Coverage Δ
aiosip/contact.py	86% <100%> (+11%)	:arrow_up:
aiosip/uri.py	71.92% <0%> (+3.5%)	:arrow_up:

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