iOS RestKit POST Response Content type - is not being set properly

Question

i set

[objectManager setAcceptHeaderWithMIMEType:@"text/json"];

& in php server script

header('content-type:text/json;charset=utf-8');

But still getting

Error: .... NSLocalizedDescription=Expected content type {(
    "application/x-www-form-urlencoded",
    "application/json"
)}, got text/json}