Fix IOPIPE_TRACE_AUTO_HTTP_ENABLED env var logic

[kolanos]

No description provided.

[codecov]

Codecov Report

Merging #332 into master will increase coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #332      +/-   ##
==========================================
+ Coverage   83.64%   83.66%   +0.02%     
==========================================
  Files          36       36              
  Lines        1357     1359       +2     
==========================================
+ Hits         1135     1137       +2     
  Misses        222      222

Impacted Files	Coverage Δ
iopipe/contrib/trace/plugin.py	100% <100%> (ø)	⬆️

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