fix(cubejs-client-core): Remove Content-Type header from requests in HttpTransport

[oguzbilgener]

Check List

• Tests has been run in packages where changes made if available
• Linter has been run for changed code
• Tests for the changes have been added if not covered yet
• Docs have been added / updated if required

Issue Reference this PR resolves

#688

Description of Changes Made

• Remove the Content-Type header in the fetch call, as explained in the issue, GET requests by @cubejs-client/core shouldn't contain a Content-Type header #688.
• Add some tests for HttpTransport.

[codecov-commenter]

Codecov Report

Merging #709 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #709   +/-   ##
=======================================
  Coverage   80.71%   80.71%           
=======================================
  Files          40       40           
  Lines        3319     3319           
=======================================
  Hits         2679     2679           
  Misses        640      640           

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