It doesn't look like RAML can validate that kind of conditions. About headers the RAML specification only says:
the value of the headers node MUST be a map, specifically a
properties declaration. Each property in this declaration object is a
header declaration. Each property name specifies an allowed header
name. Each property value specifies the header value type as a type
name or an inline type declaration.
Assuming that you are implementing this RAML API in Mule runtime with APIKit then you will need to implement the restriction in the body of the flow.
It doesn't look like RAML can validate that kind of conditions. About headers the RAML specification only says:
Assuming that you are implementing this RAML API in Mule runtime with APIKit then you will need to implement the restriction in the body of the flow.