YCP AST: Small adjustments of formatting and comment
Ruby AST specs: Add missing context-passing specs for |Hash|
Ruby AST specs: Remove |[...].join("\n")| for one-line strings
Ruby AST specs: s/check_context/check_to_ruby_context/

This is a preparation for a change where I'll introduce checking also
for context passed to |single_line_width|.
Ruby AST specs: Add context-passing specs for |single_line_width_base|