"timeColumn" option not respected in a "read.dataframe" call

@icexelloss hi there!

I'm glad the issues are being (pro)actively monitored and attended to, I wasn't expecting that.

Here is one issue I'm facing, it's not a big one, but an inconvenient one:
```
print( sc.version )
print( tm )

n = df.filter( df['Container'] == 'dbc94d4e3af6' ).select( tm, 'MemPercentG', 'CpuPercentG' )
n.show( truncate = False )
n.printSchema()

from ts.flint import FlintContext, clocks
from ts.flint import utils

fc = FlintContext( sqlContext )

r = fc.read \
.option('isSorted', False) \
.option('timeUnit', 's') \
.option('timeColumn', tm) \
.dataframe( n )
```

The output is:
```
2.3.1
TimeStamp
+-------------------+---------------+------------+
|TimeStamp |MemPercentG |CpuPercentG |
+-------------------+---------------+------------+
|2018-08-01 05:55:35|0.0030517578125|0.002331024 |
|2018-08-01 05:58:05|0.0030517578125|0.0031538776|
|2018-08-01 05:59:05|0.0030517578125|0.0030176123|
+-------------------+---------------+------------+

root
|-- TimeStamp: timestamp (nullable = true)
|-- MemPercentG: double (nullable = true)
|-- CpuPercentG: float (nullable = true)

IllegalArgumentException: 'Field "time" does not exist.\nAvailable fields: TimeStamp, MemPercentG, CpuPercentG'
---------------------------------------------------------------------------
IllegalArgumentException Traceback (most recent call last)
in ()
14 fc = FlintContext( sqlContext )
15
---> 16 r = fc.read .option('isSorted', False) .option('timeUnit', 's') .option('timeColumn', tm) .dataframe( n )
17
18

/databricks/python/lib/python3.5/site-packages/ts/flint/readwriter.py in dataframe(self, df, begin, end, timezone, is_sorted, time_column, unit)
362 time_column=time_column,
363 is_sorted=is_sorted,
--> 364 unit=self._parameters.timeUnitString())
365
366 def parquet(self, *paths):

/databricks/python/lib/python3.5/site-packages/ts/flint/dataframe.py in _from_df(df, time_column, is_sorted, unit)
248 time_column=time_column,
249 is_sorted=is_sorted,
--> 250 unit=unit)
251
252 @staticmethod

/databricks/python/lib/python3.5/site-packages/ts/flint/dataframe.py in __init__(self, df, sql_ctx, time_column, is_sorted, unit, tsrdd_part_info)
133 # throw exception
134 if time_column in df.columns:
--> 135 self._jdf = self._jpkg.TimeSeriesRDD.canonizeTime(self._jdf, self._junit)
136
137 if tsrdd_part_info:

/databricks/spark/python/lib/py4j-0.10.7-src.zip/py4j/java_gateway.py in __call__(self, *args)
1255 answer = self.gateway_client.send_command(command)
1256 return_value = get_return_value(
-> 1257 answer, self.gateway_client, self.target_id, self.name)
1258
1259 for temp_arg in temp_args:

/databricks/spark/python/pyspark/sql/utils.py in deco(*a, **kw)
77 raise QueryExecutionException(s.split(': ', 1)[1], stackTrace)
78 if s.startswith('java.lang.IllegalArgumentException: '):
---> 79 raise IllegalArgumentException(s.split(': ', 1)[1], stackTrace)
80 raise
81 return deco

IllegalArgumentException: 'Field "time" does not exist.\nAvailable fields: TimeStamp, MemPercentG, CpuPercentG'
```

Is this reproducible for you?

Please, advise, if I'm not using/calling it correctly or if it's a bug.

The flint libraries (the Scala and the Python ones) I installed on DataBricks via its UI (from the respective online repos, which might be dated) - I can try and install the latest builds from the freshest source code, if you think that will help.

Thanks.