code: public CustomHeadersInterceptor replaceHeader(String name, String value) {
          this.headers.put(name, new ArrayList<String>());
          this.headers.get(name).add(value);
          return this;
      }
docs: Add a single header key-value pair. If one with the name already exists,
  it gets replaced.
  
  @param name the name of the header.
  @param value the value of the header.
  @return the interceptor instance itself.