fix tests on Edge for ReadableStream

[gfx]

Fix https://travis-ci.org/msgpack/msgpack-javascript/jobs/539247973

Edge 18.17763.0 (Windows 10.0.0) whatwg streams decodeAsync FAILED
TypeError: Function expected
at Anonymous function (webpack:///test/whatwg-streams.test.ts:14:10 <- test/karma-run.js:26328:21)

This PR also fixes codecov settings and now it gets sane.

[codecov-io]

Codecov Report

Merging #49 into master will increase coverage by 3.12%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master      #49      +/-   ##
==========================================
+ Coverage   93.45%   96.57%   +3.12%     
==========================================
  Files          15       15              
  Lines         840      906      +66     
  Branches      175      183       +8     
==========================================
+ Hits          785      875      +90     
- Misses         29       31       +2     
+ Partials       26        0      -26

Impacted Files	Coverage Δ
src/timestamp.ts	100% <0%> (ø)	⬆️
src/wasmFunctions.ts	94.23% <0%> (+0.35%)	⬆️
src/Decoder.ts	97.4% <0%> (+1.52%)	⬆️
src/Encoder.ts	97.8% <0%> (+3.71%)	⬆️
src/ExtensionCodec.ts	100% <0%> (+5.55%)	⬆️
src/utils/utf8.ts	96.47% <0%> (+8.23%)	⬆️
src/utils/stream.ts	40% <0%> (+13.33%)	⬆️
src/utils/typedArrays.ts	81.81% <0%> (+18.18%)	⬆️

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[gfx]

This PR only includes test fixes, so I'll merge it after Ci passes.

cc; @sergeyzenchenko