Add missing headers

[yihezkel]

Changed

• Include "charset=utf-8" in HttpHeaders.CONTENT_TYPE because that's best practice
• Remove "Fed" header, because it's to indicate "AAD federated authentication", but authentication should be dependent on the KCSB (see when FEDERATED_SECURITY_SUFFIX is stripped in com.microsoft.azure.kusto.data.ClientImpl.ClientImpl(ConnectionStringBuilder, CloseableHttpClient, boolean))

[github-actions[bot]]

Test Results

311 tests  ±0   305 :white_check_mark: ±0   4h 33m 34s :stopwatch: + 32m 23s  24 suites ±0     6 :zzz: ±0   24 files   ±0     0 :x: ±0 

Results for commit b1835b18. ± Comparison against base commit 09b65d03.

:recycle: This comment has been updated with latest results.

[codecov[bot]]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (09b65d0) 56.61% compared to head (b1835b1) 56.63%.

Files	Patch %	Lines
...ava/com/microsoft/azure/kusto/data/ClientImpl.java	0.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #342 +/- ## ============================================ + Coverage 56.61% 56.63% +0.01% Complexity 811 811 ============================================ Files 109 109 Lines 4094 4093 -1 Branches 443 443 ============================================ Hits 2318 2318 + Misses 1582 1581 -1 Partials 194 194 ```

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[AsafMah]

Why did you add the query_language header? we don't see it used anywhere else

[yihezkel]

Why did you add the query_language header? we don't see it used anywhere else

I see from the traces many clients that send this. But I don't think it helps anything, so I'm removing it for now.

[The-Funk]

I moved the addCommandHeaders logic in PR #335 to a request builder. I've adjusted the builder to make the same changes that were made here and resolved conflicts.