private RequestTemplate appendHeader(String name, Iterable<String> values) {
    return this.appendHeader(name, values, false);
  }
Append the Header. Will create a new Header if it doesn't already exist. Treats all values as
potentially expressions.

@param name of the header
@param values for the header, may be expressions.
@return a RequestTemplate for chaining.
core/src/main/java/feign/RequestTemplate.java