code: public void setDefaultHeader(String name, Object value) {
          if (defaultHeaders == null) {
              defaultHeaders = new HashMap<String, Object>();
          }
          if (value == null) {
              defaultHeaders.remove(name);
          } else {
              defaultHeaders.put(name, value);
          }
      }
docs: Set the value for a named header which is valid for all requests created by this instance.
  <br>
  The value takes precedence over {@link Webb#setGlobalHeader(String, Object)} but can be overwritten by
  {@link com.goebl.david.Request#header(String, Object)}.
  <br>
  For the supported types for values see {@link Request#header(String, Object)}.
  
  @param name name of the header (regarding HTTP it is not case-sensitive, but here case is important).
  @param value value of the header. If <code>null</code> the header value is cleared (effectively not set).
  When setting the value to null, a value from global headers can shine through.
  
  @see #setGlobalHeader(String, Object)
  @see com.goebl.david.Request#header(String, Object)