Update JsonGzipHttpClient.cs

[AntonSmolkov]

Add data flushing from gzipStream to underlying stream.

MSDN says:

The write operation might not occur immediately but is buffered until the buffer size is reached or until the Flush or Close method is called.

C# 8 using var does not make it flushed automatically because it

tells the compiler that the variable being declared should be disposed at the end of the enclosing scope

Description

Fixes #190