## What changes were proposed in this pull request?
Add implicit encoders for BigDecimal, timestamp and date.
## How was this patch tested?
Add an unit test. Pass build, unit tests, and some tests below .
Before:
```
scala> spark.createDataset(Seq(new java.math.BigDecimal(10)))
<console>:24: error: Unable to find encoder for type stored in a Dataset. Primitive types (Int, String, etc) and Product types (case classes) are supported by importing spark.implicits._ Support for serializing other types will be added in future releases.
spark.createDataset(Seq(new java.math.BigDecimal(10)))
^
scala>
```
After:
```
scala> spark.createDataset(Seq(new java.math.BigDecimal(10)))
res0: org.apache.spark.sql.Dataset[java.math.BigDecimal] = [value: decimal(38,18)]
```
Author: Weiqing Yang <yangweiqing001@gmail.com>
Closes#16176 from weiqingy/SPARK-18746.
ae5b2d3e46