JIRA: https://issues.apache.org/jira/browse/SPARK-7939

Author: Liang-Chi Hsieh <viirya@gmail.com>

Closes #6503 from viirya/disable_partition_type_inference and squashes the following commits:

3e90470 [Liang-Chi Hsieh] Default to enable type inference and update docs.
455edb1 [Liang-Chi Hsieh] Merge remote-tracking branch 'upstream/master' into disable_partition_type_inference
9a57933 [Liang-Chi Hsieh] Add conf to enable/disable partition column type inference.