[SPARK-4928][SQL] Fix: Operator '>,<,>=,<=' with decimal between different precision report error

case operator  with decimal between different precision, we need change them to unlimited

Author: guowei2 <guowei2@asiainfo.com>

Closes #3767 from guowei2/SPARK-4928 and squashes the following commits:

c6a6e3e [guowei2] fix code style
3214e0a [guowei2] add test case
b4985a2 [guowei2] fix code style
27adf42 [guowei2] Fix: Operation '>,<,>=,<=' with Decimal report error