config-servlet/src/main/java/org/ocpsoft/rewrite/servlet/config/Header.java
public static Header valueExists(final String value)
   {
      return new Header("{" + Header.class.getName() + "_name}", value) {
         @Override
         public String toString()
         {
            return "Header.valueExists(\"" + value + "\")";
         }
      };
   }
public static Header valueExists(final String value)
   {
      return new Header("{" + Header.class.getName() + "_name}", value) {
         @Override
         public String toString()
         {
            return "Header.valueExists(\"" + value + "\")";
         }
      };
   }
Create a {@link Header} condition that matches only against the existence of a header with value matching the
given pattern. The header name is ignored.
<p>
The given value pattern may be parameterized:
<p>
<code>
Header.valueExists("application/xml")<br>
Header.valueExists("application/{type}")<br>
Header.valueExists("{anything}")<br>
...
</code>
<p>

@see {@link ConfigurationRuleParameterBuilder#where(String) {@link HttpServletRequest#getHeader(String)}

@param value {@link ParameterizedPattern} matching the header value.