Use default content-type in vespa-fbench for post queries

[frodelu]

When using vespa-fbench, you need to specifically add the content-type header (-H "Content-type:application/json") when using HTTP POST queries.

When you specify post queries on the command line with the -P option, we should add the relevant content-type header as default unless another one is explicitly set with the -H option.

[frodelu]

Alternative to defaulting to json content-type is to at least require a content-type explicitly specified (using -H option) if you use post requests (with the -P option).

[baldersheim]

fixed in vespa-8.34.21