Content Type header shouldn't be dropped in `@head` operations

[timotheeguerin]

@head
op head(): { @header "content-type": "text/plain" };

in this case the getHttpOperation will not return the header and log a warning that it is ignore. It shouldn't as it is allowed in head operations

From Azure/typespec-azure#3975

[markcowl]

Also, should not emit the "@typespec/http/content-type-ignored" diagnostic in this case