Upgrade OkHTTP 4.10 -> 4.12 fixes klippa-app/nativescript-http#103

[vpiskunov]

PR Checklist

• The PR title follows our guidelines: https://github.com/NativeScript/NativeScript/blob/master/CONTRIBUTING.md#commit-messages.
• There is an issue for the bug/feature this PR is for. To avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it.
• All existing tests are passing
• Tests for the changes are included (N/A)

What is the current behavior?

Some requests fail with okhttp3.internal.http2.StreamResetException: stream was reset: INTERNAL_ERROR.

What is the new behavior?

Using later version of OkHTTP, requests succeed.

Fixes/Implements/Closes #103 .