[python] check output content-type only if preload_content is set

[tomplus]

In the Python client generator we have code to detect output encoding (#5679). This code checks received headers but it doesn't work if preloading content is disabled. In such situation raw response is returned (from native client library like aiohttp) and method getheader() doesn't exist. In this PR I rearrange code a little bit to work in such cases.

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CC: @taxpon (2017/07) @frol (2017/07) @mbohlool (2017/07) @cbornet (2017/09) @kenjones-cisco (2017/11) @tomplus (2018/10) @Jyhess (2019/01) @arun-nalla (2019/11) @spacether (2019/11)

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[kenjones-cisco]

LGTM!

[spacether]

Looks great! Thank you for your PR!