[BitBucket] Fix get_project_tags.

[Spacetown]

If tag_name is given a single tag is returned, else a generator for the tags (paged API).

Remove ifs in set_tag because the tag_name is mandatory, description is the only optional argument.

Closes #827

[codecov-commenter]

Codecov Report

Merging #829 (9b8d4d7) into master (e5bff44) will increase coverage by 0.01%.
The diff coverage is 16.66%.

@@            Coverage Diff             @@
##           master     #829      +/-   ##
==========================================
+ Coverage   35.73%   35.75%   +0.01%     
==========================================
  Files          32       32              
  Lines        6150     6147       -3     
  Branches      890      889       -1     
==========================================
  Hits         2198     2198              
+ Misses       3854     3851       -3     
  Partials       98       98              

Impacted Files	Coverage Δ
atlassian/bitbucket/__init__.py	28.78% <16.66%> (+0.08%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update e5bff44...9b8d4d7. Read the comment docs.