Make \def, \let, and \newcommand handle '\ ' properly.  Make \let set a control sequence to an undefined control sequence.  Resolves issues #1563 and #1564.
Make \def, \let, and \newcommand handle '\ ' properly.  Make \let set a control sequence to an undefined control sequence.  Resolves issues #1563 and #1564.

dpvc/MathJax,wikimedia/MathJax,GerHobbelt/MathJax,jinntrance/mathjax,jinntrance/mathjax,GerHobbelt/MathJax