add meteor support for xmlhttprequest

[zachferland]

Meteor builds include node-xmltthprequest rather than using window.xmlhttprequest. node-xmltthprequest has a bug in it, and doesn't seem to be pulling in PRs. Meteor builds use node-xmltthprequest since it ignores the browser field in the package.json, unless it is just a string - meteor/meteor#6890 (comment) . These changes allows meteor to use window.xmlhttprequest. (also reduces code, and does not affect local webpack builds)

[codecov-io]

Codecov Report

Merging #3 into develop will decrease coverage by -0.19%.
The diff coverage is 100%.

@@             Coverage Diff             @@
##           develop       #3      +/-   ##
===========================================
- Coverage    95.83%   95.65%   -0.19%     
===========================================
  Files            2        1       -1     
  Lines           48       46       -2     
  Branches        14       15       +1     
===========================================
- Hits            46       44       -2     
  Misses           2        2

Impacted Files	Coverage Δ
src/index.js	95.65% <100%> (ø)	✅

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[pelle]

Looks great. Thankyou @zachferland