Fix is_friend bug & improve docstring of contact module

[wj-Mcat]

Description

• [x] fix #317
• [x] improve docstring(google type) of contact module.

[codecov[bot]]

Codecov Report

Merging #318 (80832a8) into master (92b0ad6) will decrease coverage by 0.12%. The diff coverage is 25.00%.

@@            Coverage Diff             @@
##           master     #318      +/-   ##
==========================================
- Coverage   40.14%   40.02%   -0.13%     
==========================================
  Files          26       26              
  Lines        1908     1914       +6     
==========================================
  Hits          766      766              
- Misses       1142     1148       +6     

Flag	Coverage Δ
unittests	40.02% <25.00%> (-0.13%)	:arrow_down:

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/wechaty/plugin.py	27.88% <0.00%> (+0.10%)	:arrow_up:
src/wechaty/user/contact_self.py	37.14% <0.00%> (ø)
src/wechaty/user/contact.py	32.83% <31.25%> (-1.19%)	:arrow_down: