modules/spring-mock-mvc/src/main/java/io/restassured/module/mockmvc/specification/MockMvcRequestSpecBuilder.java
public MockMvcRequestSpecBuilder addHeader(String headerName, String headerValue) {
        spec.header(headerName, headerValue);
        return this;
    }
public MockMvcRequestSpecBuilder addHeader(String headerName, String headerValue) {
        spec.header(headerName, headerValue);
        return this;
    }
Add a header to be sent with the request

@param headerName  The header name
@param headerValue The header value
@return The request specification builder