[SPARK-8840] [SPARKR] Add float coercion on SparkR

JIRA: https://issues.apache.org/jira/browse/SPARK-8840

Currently the type coercion rules don't include float type. This PR simply adds it.

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

Closes #7280 from viirya/add_r_float_coercion and squashes the following commits:

c86dc0e [Liang-Chi Hsieh] For comments.
dbf0c1b [Liang-Chi Hsieh] Implicitly convert Double to Float based on provided schema.
733015a [Liang-Chi Hsieh] Add test case for DataFrame with float type.
30c2a40 [Liang-Chi Hsieh] Update test case.
52b5294 [Liang-Chi Hsieh] Merge remote-tracking branch 'upstream/master' into add_r_float_coercion
6f9159d [Liang-Chi Hsieh] Add another test case.
8db3244 [Liang-Chi Hsieh] schema also needs to support float. add test case.
0dcc992 [Liang-Chi Hsieh] Add float coercion on SparkR.