code: public void setRequestProperty(String pKey, String pValue) {
          if (connected) {
              throw new IllegalAccessError("Already connected");
          }
          String oldValue = requestProperties.getProperty(pKey);
  
          if (oldValue == null) {
              requestProperties.setProperty(pKey, pValue);
          }
          else {
              requestProperties.setProperty(pKey, oldValue + ", " + pValue);
          }
      }
docs: Sets the general request property. If a property with the key already
  exists, overwrite its value with the new value.
  <p/>
  <p> NOTE: HTTP requires all request properties which can
  legally have multiple instances with the same key
  to use a comma-seperated list syntax which enables multiple
  properties to be appended into a single property.
  
  @param pKey   the keyword by which the request is known
  (e.g., "{@code accept}").
  @param pValue the value associated with it.
  @see #getRequestProperty(java.lang.String)