Fix gzip modifies headers

[OrkhanAlikhanov]

Asked sonnet 4.5 to fix #133

From my review the code is legit, it only sets headers only if it goes ahead with compression (e.i g.writer.Write call)

Edit:

• I had to fix another issue where when the upstream gzip was sent in chunks it was correctly passing through the first chunk but was compressing the subsequent ones, thus resulting in invalid data.
• Also removed the code where it sets "Content-Length" in defer statement, it does not make sense to set header after body was already written. It was basically no-op.
• Sorry for kinda major rework but test cases should be universal. They do not pass in v1.2.5