[SPARK-5203][SQL] fix union with different decimal type

   When union non-decimal types with decimals, we use the following rules:
      - FIRST `intTypeToFixed`, then fixed union decimals with precision/scale p1/s2 and p2/s2  will be promoted to
      DecimalType(max(p1, p2), max(s1, s2))
      - FLOAT and DOUBLE cause fixed-length decimals to turn into DOUBLE (this is the same as Hive,
      but note that unlimited decimals are considered bigger than doubles in WidenTypes)

Author: guowei2 <guowei2@asiainfo.com>

Closes #4004 from guowei2/SPARK-5203 and squashes the following commits:

ff50f5f [guowei2] fix code style
11df1bf [guowei2] fix decimal union with double, double->Decimal(15,15)
0f345f9 [guowei2] fix structType merge with decimal
101ed4d [guowei2] fix build error after rebase
0b196e4 [guowei2] code style
fe2c2ca [guowei2] handle union decimal precision in 'DecimalPrecision'
421d840 [guowei2] fix union types for decimal precision
ef2c661 [guowei2] fix union with different decimal type