[SPARK-14189][SQL] JSON data sources find compatible types even if inferred decimal type is not capable of the others

## What changes were proposed in this pull request?

https://issues.apache.org/jira/browse/SPARK-14189

When inferred types in the same field during finding compatible `DataType`, are `IntegralType` and `DecimalType` but `DecimalType` is not capable of the given `IntegralType`, JSON data source simply fails to find a compatible type resulting in `StringType`.

This can be observed when `prefersDecimal` is enabled.

```scala
def mixedIntegerAndDoubleRecords: RDD[String] =
  sqlContext.sparkContext.parallelize(
    """{"a": 3, "b": 1.1}""" ::
    """{"a": 3.1, "b": 1}""" :: Nil)

val jsonDF = sqlContext.read
  .option("prefersDecimal", "true")
  .json(mixedIntegerAndDoubleRecords)
  .printSchema()
```

- **Before**

```
root
 |-- a: string (nullable = true)
 |-- b: string (nullable = true)
```

- **After**

```
root
 |-- a: decimal(21, 1) (nullable = true)
 |-- b: decimal(21, 1) (nullable = true)
```
(Note that integer is inferred as `LongType` which becomes `DecimalType(20, 0)`)

## How was this patch tested?

unit tests were used and style tests by `dev/run_tests`.

Author: hyukjinkwon <gurwls223@gmail.com>

Closes #11993 from HyukjinKwon/SPARK-14189.