fix(catalog/rest): Do not set Content-type header in doPost

[curtisr7]

This change shouldn't really be necessary..... but I just spent the better part of an afternoon tracking it down.

I've been testing against snowflake opencatalog and it appears that they're running some older version of polaris where a bug exists where if you specify a Content-Type header multiple times on a POST you'll get a cryptic 400 back from the server. I wasn't able to find a bug in apache/polaris, but the problem doesn't happen on main.

Seems safe to do as the header is defaulted here.

iceberg-go/rgc % curl -X GET -H "Authorization: Bearer xxx" \ 
        -H 'Accept: application/json' \
        -H 'Accept: application/json' \
        -H 'Content-Type: application/json' \
        -H 'Content-Type: application/json' \
        https:/*******.snowflakecomputing.com/polaris/api/catalog/v1/test-catalog/namespaces
{
"servlet":"com.sun.jersey.spi.container.servlet.ServletContainer-16626b18",
"message":"Bad Request",
"url":"/polaris/api/catalog/v1/test-catalog/namespaces",
"status":"400"

[zeroshade]

Well, that's annoying, but makes sense. LGTM