Use string interpolation for trailing whitespace

Code editors are commonly configured to strip trailing whitespace when
saving.  Additionally, trailing whitespace may be hard to distinguish
visually, both in the editor and in diffs.

This commit uses string interpolation to encode expected trailing
whitespace.

Author: jonathanhefner

spec/shell/basic_spec.rb

@@ -321,7 +321,7 @@ def  #456  Lanç...
       2.times { @table.first.pop }
       content = capture(:stdout) { shell.print_table(@table) }
       expect(content).to eq(<<-TABLE)
-abc  
+abc#{"  "}
      #0    empty
 xyz  #786  last three
 TABLE