JSON-RPC request Content-Type is application/json

[luke-jr]

This goes all the way back to #3246, and it's not clear why text/plain has been used throughout.

There seems to be a lack of specification of what the correct content-type is, but application/json seems correct and popular, and text/plain definitely isn't.

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Conflicts

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• #27101 (Support JSON-RPC 2.0 when requested by client by pinheadmz)

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[laanwj]

IIRC we do return application/json content type, so this at least makes it symmetric. id be against enforcing this (because so many clients already specify it wrong), but the example would be better this way.

[fanquake]

Removed bugfix given this is a doc/test only change.

[laanwj]

Code review ACK f90a84d