Fix bugs in space stripping in Weather.NOAA

`raw_to_xml/1` did not remove empty text nodes because I used an Elixir
String, " ", instead of an Erlang string, ' '.  Found the bug by
increasing the code coverage.  Even with the fix to `raw_to_xml/1`,
leading and trailing spaces remained for text nodes, so strip the String
before returning from `find_text/2`.