Don't add parentheses when splitting symbols to objects in dictionaries.

Crucial when object is a function so that it is not multiplied by its argument.

c.f. https://groups.google.com/forum/#!topic/sympy/MKhZdgB_Sg4
IMO, parentheses should override symbol splitting in precedence. So in this case, although you can split symbols the parentheses should cause the function to be caught rather than to interpret it as a symbol. To disambiguate, the user would have to put a `*` before the parentheses. Those who do more parsing can probably tell me why that might be a bad idea, if it is.