Add extra test for IO.new with internal_encoding keyword

When providing a single encoding in the mode argument, the external
encoding of the IO object is set to that encoding, the internal encoding
is left blank. The ArgumentError in this case is a rather specific
behaviour.

Author: herwinw

core/io/shared/new.rb

@@ -324,6 +324,9 @@
     -> {
       @io = IO.send(@method, @fd, 'w:ISO-8859-1', external_encoding: 'ISO-8859-1')
     }.should raise_error(ArgumentError)
+    -> {
+      @io = IO.send(@method, @fd, 'w:ISO-8859-1', internal_encoding: 'ISO-8859-1')
+    }.should raise_error(ArgumentError)
     -> {
       @io = IO.send(@method, @fd, 'w:ISO-8859-1:UTF-8', internal_encoding: 'ISO-8859-1')
     }.should raise_error(ArgumentError)