Avoid copying FieldStorage if possible.

[benjaminp]

On Python 3, cgi.FieldStorage has a del method that closes the underlying file [1]. This means that if the copy made from UnicodeMultiDict._decode_value is garbage collected, the file underlying the original FieldStorage will be closed! Fix this by not copying FieldStorage if it is not required by decode_keys=False. I cannot think of a nice way to fix this problem if decode_keys=True.

[1] python/cpython@f79126f

[codecov-io]

Codecov Report

Merging #30 into master will increase coverage by <.01%.
The diff coverage is 93.33%.

@@            Coverage Diff             @@
##           master      #30      +/-   ##
==========================================
+ Coverage   53.74%   53.75%   +<.01%     
==========================================
  Files         125      125              
  Lines       14785    14790       +5     
  Branches     2499     2500       +1     
==========================================
+ Hits         7946     7950       +4     
+ Misses       6288     6287       -1     
- Partials      551      553       +2

Flag	Coverage Δ
#py27	52.69% <86.66%> (ø)	⬆️
#py35	52.03% <60%> (ø)	⬆️
#py36	52.03% <60%> (ø)	⬆️
#py37	52.03% <60%> (ø)	⬆️
#pypy	51.94% <86.66%> (ø)	⬆️

Impacted Files	Coverage Δ
tests/test_multidict.py	100% <100%> (ø)	⬆️
paste/util/multidict.py	81.67% <85.71%> (-0.26%)	⬇️
paste/progress.py	0% <0%> (ø)	⬆️

---

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[codecov-io]

Codecov Report

Merging #30 into master will increase coverage by <.01%.
The diff coverage is 93.33%.

@@            Coverage Diff             @@
##           master      #30      +/-   ##
==========================================
+ Coverage   53.74%   53.75%   +<.01%     
==========================================
  Files         125      125              
  Lines       14785    14790       +5     
  Branches     2499     2500       +1     
==========================================
+ Hits         7946     7950       +4     
+ Misses       6288     6287       -1     
- Partials      551      553       +2

Flag	Coverage Δ
#py27	52.69% <86.66%> (ø)	⬆️
#py35	52.03% <60%> (ø)	⬆️
#py36	52.03% <60%> (ø)	⬆️
#py37	52.03% <60%> (ø)	⬆️
#pypy	51.94% <86.66%> (ø)	⬆️

Impacted Files	Coverage Δ
tests/test_multidict.py	100% <100%> (ø)	⬆️
paste/util/multidict.py	81.67% <85.71%> (-0.26%)	⬇️
paste/progress.py	0% <0%> (ø)	⬆️

---

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Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 9eff34d...b1f9227. Read the comment docs.

[cdent]

🙈

I'll get to this soon. It's going to take a bit of mental effort to register properly.

[cdent]

Let me know if you think you'll have a few more of these queued up and I'll hold off on doing a release. Otherwise I'll do one soon.