public String getMappedRequestHeaders() {
		return mappedRequestHeaders;
	}
Base class for mixins that add the {@code mappedRequestHeaders} option.
Implemented as a base abstract class (with some concrete implementations as static inner classes)
because the default value for the option varies by adapter.

 @author Eric Bottard
spring-xd-module-spi/src/main/java/org/springframework/xd/module/options/mixins/MappedRequestHeadersMixin.java