Add test to increase test coverage on config.py

[zkan]

Fixes #420

Checklist

• I have made a material change to the repo (functionality, testing, spelling, grammar)
• I have read the [Contribution Guide] and my PR follows them.
• I updated my branch with the master branch.
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation about the functionality in the appropriate .md file
• I have added in line documentation to the code I modified

Short description of what this PR does:

• Add test to check that it should not set env when env format is invalid.

If you have questions, please send an email to SendGrid, or file a GitHub Issue in this repository.

[codecov]

Codecov Report

Merging #643 into master will increase coverage by 0.08%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #643      +/-   ##
==========================================
+ Coverage   84.77%   84.86%   +0.08%     
==========================================
  Files          35       35              
  Lines        1156     1156              
  Branches      172      172              
==========================================
+ Hits          980      981       +1     
  Misses         90       90              
+ Partials       86       85       -1

Impacted Files	Coverage Δ
sendgrid/helpers/inbound/config.py	96.96% <0%> (+3.03%)	⬆️

---

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 1627df7...2ba4666. Read the comment docs.

[thinkingserious]

Hello @zkan,

Thanks again for the PR!

It's HACKTOBERFEST! We want to show our appreciation by sending you some special Hacktoberfest swag. If you have not already, could you please fill out this form so we can send it to you? Thanks!

Team SendGrid DX

[zkan]

@thinkingserious Thank you!