in_splunk: Fix handlings for content type when http2 is off

[cosmo0920]

Don't respond twice for Content-Type is not set.
Also, Splunk's official documentation does not mention to set Content-Type: application/json or Content-Type: text/plain.
So, we need to pass-through the request which don't use Content-Type header.

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[lecaros]

The behavior is the expected cc @edsiper
Thanks @cosmo0920!