net: lwm2m: check content-format and respond with error if not support
net: lwm2m: check content-format and respond with error if not support

Currently, we always set the content-format as "plain/text" when
it is unrecognized. This is wrong for it's possible that payload
is actually something else.

For example, we don't support JSON as incoming format right now.
But if I send a PUT request to /1/0/1 (server objectinstance/lifetime
resource) with value 3200 in JSON format: {"e":[{"n":"","v":3200}]}.
The client will still handle the request and respond with changed (2.04)
except the lifetime resource is updated incorrectly due to parsing
error.

Correct the behavior by not setting a default format and respond with
content-format-not-support error code (4.15)

Signed-off-by: Robert Chou <563fc56b80720efb29939356d471536b8c17d514@acer.com>

nashif/zephyr,punitvara/zephyr,zephyriot/zephyr,ldts/zephyr,finikorg/zephyr,Vudentz/zephyr,punitvara/zephyr,GiulianoFranchetto/zephyr,zephyrproject-rtos/zephyr,kraj/zephyr,explora26/zephyr,galak/zephyr,kraj/zephyr,GiulianoFranchetto/zephyr,finikorg/zephyr,mbolivar/zephyr,galak/zephyr,mbolivar/zephyr,Vudentz/zephyr,nashif/zephyr,finikorg/zephyr,kraj/zephyr,ldts/zephyr,punitvara/zephyr,GiulianoFranchetto/zephyr,kraj/zephyr,zephyriot/zephyr,zephyrproject-rtos/zephyr,ldts/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,zephyriot/zephyr,galak/zephyr,punitvara/zephyr,mbolivar/zephyr,GiulianoFranchetto/zephyr,galak/zephyr,GiulianoFranchetto/zephyr,nashif/zephyr,explora26/zephyr,Vudentz/zephyr,Vudentz/zephyr,kraj/zephyr,ldts/zephyr,zephyriot/zephyr,explora26/zephyr,Vudentz/zephyr,nashif/zephyr,finikorg/zephyr,zephyriot/zephyr,ldts/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,mbolivar/zephyr,galak/zephyr,punitvara/zephyr,explora26/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,mbolivar/zephyr,explora26/zephyr