Code cleaning and Javadoc

TikaResource Uses  String key : httpHeaders.keySet() to iterate over header keys, and then inside processHeaderConfig(...), the first header value is fetched, and stored in "val" variable, for further usage.

As, this method (processHeaderConfig) should be logically called (and practically) per header value pair, it would be cleaner to extract the header value to outside.

Additionally, few missing javadocs added. Updated javadocs for changed methods as well.