[SPARK-20948][SQL] Built-in SQL Function UnaryMinus/UnaryPositive support string type

[wangyum]

What changes were proposed in this pull request?

Built-in SQL Function UnaryMinus/UnaryPositive support string type, if it's string type, convert it to double type, after this PR:

spark-sql> select positive('-1.11'), negative('-1.11');
-1.11   1.11
spark-sql> 

How was this patch tested?

unit tests

[SparkQA]

Test build #77635 has finished for PR 18173 at commit 5e7ed08.

• This patch fails Spark unit tests.
• This patch merges cleanly.
• This patch adds no public classes.

[SparkQA]

Test build #77638 has finished for PR 18173 at commit 3cc6f65.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[SparkQA]

Test build #78173 has finished for PR 18173 at commit 7247282.

• This patch fails Spark unit tests.
• This patch merges cleanly.
• This patch adds no public classes.

[SparkQA]

Test build #78175 has finished for PR 18173 at commit 484a353.

• This patch fails Spark unit tests.
• This patch merges cleanly.
• This patch adds no public classes.

[SparkQA]

Test build #78195 has finished for PR 18173 at commit fd279b7.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[wangyum]

@gatorsmile It's same to ABS function support string type.

[gatorsmile]

LGTM

[gatorsmile]

Thanks! Merging to master.